A system and method for conditionally diagnosing, assessing and tracking the symptoms of patients having mental or physical disorders involves providing an interview instrument having diagnostic and assessment information for a selected disorder. A clinician conducts a first interview of a patient to identify and assess the patient's symptoms. The clinician records the symptoms and assessment on the interview instrument. Alternatively, the patient records symptoms on the interview instrument and the clinician then conducts a first interview. The clinician develops a diagnosis and treatment plan and records them on the instrument. The clinician conducts one or more subsequent interviews of the patient for identifying and assessing the patient's symptoms and records any changes in the symptoms on the interview instrument for documenting the effects of the treatment plan. The interview instrument includes a joint use portion having a list of symptoms and assessment criteria and spaces for recording the patient's symptoms and an assessment, a diagnostic portion having predetermined, recognized diagnostic criteria for use by the clinician, a patient-use portion having educational information for use by the patient and a follow-up portion for tracking the effects of a treatment plan across multiple subsequent visits.