The investigators propose to make the first observations of LFMS treating a population of subjects with PTSD. A positive outcome for this study could translate directly into a new treatment modality for symptoms of PTSD in both acute and chronic situations. The investigator's goal is to demonstrate the safety and efficacy of LFMS as a possible aid in the treatment of PTSD.

The Low Field Magnetic Stimulation (LFMS) procedure is an application of a series of electromagnetic pulses to the brain lasting twenty minutes. The field and timing parameters of the LFMS pulses, such as pulse timing, duration, frequency, and electric and magnetic field distribution and direction are different from other neurostimulation methods. LFMS electromagnetic fields are significantly weaker than those occurring in electroconvulsive therapy (ECT) and repeated transcranial magnetic stimulation (rTMS).

LFMS was discovered at McLean Hospital and has been studied as an experimental antidepressant treatment at McLean. Results from the investigator's single-visit protocol (2006-P-001655) are encouraging, and demonstrate an immediate mood improvement from LFMS in depressed subjects. This indicates that LFMS continues to show potential as a treatment for depression. The profile of clinical response to LFMS that was observed in that study included improvement in symptoms of anxiety. In this current proposal the investigator's will study the effects of LFMS on these symptoms of depression and anxiety that are present in a population with a primary diagnosis of PTSD. The proposed protocol involves three treatments on consecutive days with follow-up ratings one week after the first treatment and follow-up by phone ratings in weeks three and four after the first treatment.

Inclusion Criteria:

1. Subjects will be men or women between the ages of 21-65.
2. Subjects will meet Diagnostics and Statistical Manual of Mental Disorders (DSM-IV) criteria for Post Traumatic Stress Disorder (PTSD), current.
3. Subject must have a PTSD Check List-5 (PCL-5) score greater than 38.
4. Subjects must be capable of providing informed consent.
5. Subjects must have an established residence and phone.
6. Subjects may be medicated or unmedicated.

Exclusion Criteria:

1. Dangerous or active suicidal ideation.
2. Pregnant or planning on becoming pregnant.
3. Substance abuse (cannot meet DSM criteria for substance abuse, no significant drug abuse within last 3 months, no major polysubstance abuse history, no history of dependence in last year, no drug use within last month).
4. Significant medical or neurological illness that might pose a risk to study enrollment or interfere with interpretation of study data.
5. Subjects must not have serious physical illnesses, neurological diseases or dementias.
6. Changes in psychiatric medication (e.g. dose or drug) within 6 weeks prior to enrollment.
7. History of an Axis 2 disorder, schizophrenia or schizoaffective disorder.
8. Contraindications for magnetic resonance imaging (MRI): Presence of a pacemaker, neurostimulator, or metal in head or neck.