This study evaluates a new investigational treatment regimen of three consecutive monthly doses of ranibizumab followed by an as-needed treatment regimen, with monthly follow-up for the first three months then two-monthly follow-up until 18 months in patients with visual impairment due to diabetic macular oedema.The aim of the study is to determine if this treatment regimen is effective and safe in these patients.

Inclusion Criteria:

• Patients with Type 1 or Type 2 diabetes mellitus.
• Patients with visual impairment due to focal or diffuse diabetic macular oedema (DMO), and not other causes, in at least one eye who are eligible for laser treatment in the opinion of the investigator.
• Best Corrected Visual Acuity between 78 and 24 letters in the study eye using Early Treatment Diabetic Retinopathy Study (ETDRS)-like grading charts (approximate Snellen equivalent of 20/32 to 20/320).
• Increased central retinal thickness which, in the opinion of the investigator, is due to DMO.

Exclusion Criteria:

• Concomitant conditions in the study eye that could prevent the improvement of visual acuity on study treatment.
• Active intraocular inflammation (grade trace or above) or infection in either eye at enrollment.
• History of uveitis in either eye at any time.
• Structural damage within 0.5 disc diameters of the centre of the macular in the study eye likely to preclude improvement in visual acuity following resolution of macular oedema.
• Planned medical or surgical intervention during the 18-month study period.
• Uncontrolled glaucoma in either eye at screening.
• Blood pressure systolic > 160 mmHg or diastolic > 100 mmHg at screening.
• Ocular conditions in the study eye that require chronic concomitant therapy with topical ocular or systemically administered corticosteroids.
• History of acute thromboembolic event within 4 months of screening.
• Untreated diabetes mellitus.
• Evidence of advanced, severe or unstable concomitant disease or its treatment which may interfere with evaluations or put the patient at special risk.

Other protocol-defined inclusion/exclusion criteria may apply