Of 26,294 consecutive patients monitored in a comprehensive drug surveillance program, 1067 (4 per cent) received methyldopa for treatment of hypertension. Adverse reactions attributed to methyldopa were reported in 149 patients (14 per cent), the most frequent being hypotension. Life-threatening adverse effects were reported in nine patients (6 per cent of reactors)--the major problems being hypotension associated in several patients with signs of cardiac or cerebral ischemia. Hypotension attributed to methyldopa was more frequent in younger patients, in those with uremia, in lighter subjects, and in those receiving a high daily dose. Marked interaction between these factors was demonstrated and eightfold differences in the frequency of hypotension were observed in different sub-groups of methyldopa recipients. Adverse effects other than hypotension were reported infrequently and did not correlate well with the previously mentioned factors. The findings suggest that methyldopa therapy should be commenced cautiously in younger patients, in the non-obese, and in those with impairment of renal function as manifest by elevated blood urea nitrogen levels.