A randomized study was carried out in 14 myasthenia patients to compare the long-term effects of two therapeutic regimens. Group I patients received prednisone 1 mg/kg/24 h for one month, then in decreasing dosage; in case of failure at the end of the first month, cyclophosphamide 2 mg/kg/24 h was added to the prednisone treatment. Group II patients received the same treatment associated with 3 plasma exchanges over a 10-day period; these were continued, if required, at the rate of once a week. The minimum follow up was one year. The results (greater improvement in muscular strength and vital capacity) were better after one month in group I and thereafter similar in both groups. However, the mean daily dose of prednisone was higher in Group I. The number of exacerbations of myasthenia was greater in group II (11 versus 2 in group I over a 24-months period). The mean fall in anti-R Ach ab was about the same in both groups. This study confirms the rapid effectiveness of plasma exchanges and their value in severe myasthenia. The higher incidence of exacerbations in Group II was probably due to a rebound phenomenon and points to the need for combined immuno-suppressive treatment.