Evaluation of therapeutic benefits from the patient's perspective is important in medical decision-making and reimbursement. This study aimed at developing and validating an instrument on patient-defined needs and benefits in dermatology. The questionnaire was developed according to international guidelines. The benefit assessment consists of two steps: before treatment, every patient defines his treatment needs according to a standardized list. After treatment, the patient rates the degree of benefits achieved. A "patient benefit index" (PBI) is calculated by averaging the preference-weighed results of all items. The PBI questionnaire was validated in a sample of 500 patients with ten skin diseases and in a treatment study on 906 patients with acne. The patients defined a broad spectrum of needs and treatment benefits, indicating disease-specific patterns. The PBI showed good feasibility, reliability (Cronbach's alpha >0.91) and construct validity, high responsiveness, and discrimination between subgroups. The PBI permits valid evaluation of patient-relevant benefits in dermatological treatment.