Patients with mild acute diverticulitis (modified Neff 0 grade), following the inclusion criteria and giving informed consent, will be included in the study protocol and will be randomly assigned to one of the treatment arms: symptomatic treatment with NSAID plus antibiotic vs symptomatic treatment with NSAID only. They will be followed-up at 48 hours, 7 days, 30 days and 3 months from the onset of the episode.

In the last years, traditional pathologic mechanisms of acute diverticulitis are being questioned and replaced by more scientifically grounded hypotheses that strongly postulate an inflammatory origin. Local pro-inflammatory cytokines, microbiota shifts, disturbed neurological intestinal signalling due to alterations in colonic neuropeptides and abnormal colonic motility are all being proposed as potential etiologic factors. Recent publications, therefore, call into question the benefits of antibiotic treatment or episodes of acute diverticulitis, especially for mild episodes. Furthermore, recent international guidelines endorse this stance in their recommendations. Moreover, recent studies provide evidence regarding the security of treating patients with mild acute diverticulitis as outpatients.

The investigators think that outpatient treatment without antibiotic for mild acute diverticulitis is not-inferior to traditional treatment with antibiotic, measuring the efficacy with readmission ratio. For this reason we have designed a multicentric, randomised, prospective study.

All patients seen in the emergency department with clinical signs of acute diverticulitis (left iliac fossa abdominal pain, peritoneal irritation signs and/or leucocytosis) will undergo an abdominal computed tomography to confirm the diagnosis and grade the disease according to severity using the modified Neff (mNeff) classification. Those with mild acute diverticulitis (grade 0), following the inclusion criteria and giving informed consent, will be included in the study protocol and will be randomly assigned to one of the treatment arms: symptomatic treatment with NSAID plus antibiotic vs symptomatic treatment with NSAID only. They will be followed-up at 48 hours, 7 days, 30 days and 3 months from the onset of the episode.

Primary goal is to determine if, in mild acute diverticulitis, the treatment without antibiotic is not-inferior to the traditional treatment with antibiotic considering readmission ratio.

Secondary goals include the analysis of the differences between groups, in case there are, in relation to number and reason for reconsultation, reason for readmission (bad symptoms control, radiologic progression, analysis worsening), pain control (analogic visual scale), recuperation after the acute episode, complication rate and their treatment.

Inclusion Criteria:

• Patient's written informed consent. Adequate cognitive capacity.
• Adequate family support
• No acute diverticulitis episode in the last 3 months
• mNeff 0 acute diverticulitis (abdominal computed tomography scan)
• No antibiotic treatment in the last 2 weeks
• Immunocompetence*
• No significant comorbidities**
• Good oral tolerance
• Good symptom control
• Maximum one of the following SIRS criteria (* T>38 ºC or <36ºC, L>12,000 or <4000/uL, HR>90 bpm, RR<20 rpm) or CRP>15 mg/dL

Exclusion Criteria:

• Women in pregnancy or breastfeeding
• Age <18 years or > 80 years.
• Absence of the patient's written informed consent. Inadequate cognitive capacity.
• Inadequate family support
• Acute diverticulitis episode in the last 3 months
• Moderate acute diverticulitis (mNeff grade I or upper)
• Antibiotic treatment in the last 2 weeks
• Inflammatory bowel disease
• Immunodepression*
• Presence of significant comorbidities**
• Bad oral tolerance
• Poor symptom control
• More than one of the following SIRS criteria (* T>38 ºC or <36ºC, L>12,000 or <4000/uL, HR>90 bpm, RR<20 rpm) or CRP>15 mg/dL

(*) Immunocompetence is the absence and immunodepression is the presence of any of the following: active neoplastic disease/hematologic malignancy/HIV with low CD4+ count/long-term corticosteroid treatment/immunosuppressant therapy/transplant/splenectomy/genetic immunodeficiency.

(**) We consider significant comorbidities any of the following: poorly controlled diabetes mellitus (HbA1>7mg/dl), cardiologic event in the last 3 months, decompensation of hepatopathy in the last 3 months, renal chronic insufficiency in dialysis programme.