A retrospective survey of the notes on all patients attending Dhoolpet Leprosy Research Center, India, during 1985 was done to establish the frequency, timing, and clinical features of reversal (type 1) reactions; 494 case notes were examined and clinical evidence of a reversal reaction was found in 44 cases (10.9%). Reactions were most common in borderline patients, with 11.4% and 14.8% of borderline tuberculoid (BT) and borderline lepromatous (BL) patients developing reactions, respectively. Presentation in reaction was frequent with 47.5% of reactional patients having signs of a reversal reaction at the time of their first visit to the Dhoolpet clinic; 50% of skin reactions developing in patients on antileprosy treatment occur within the first month of treatment. Neurological reactions occur later and over a longer time course. Late reactions may occur up to 6 1/2 years after the start of treatment. Further reactional episodes occurred in 31.8% of the patients, and may be repeated. Steroid treatment produced improvement of both skin lesions and neuritis, but improvement in clinical signs and symptoms occurred in only 50% of the neuritic episodes.