The medical records of patients presenting to the Hammersmith Hospital hypertension clinic between 1971 and 1981 were examined to determine presenting clinical data, treatment regimes, and both cardiovascular and non cardiovascular mortality and morbidity. When compared with 1004 patients receiving treatment other than hydralazine 310 patients on hydralazine had a significantly higher risk of developing renal disease (RR = 2.71) in men, and severe weight loss in women (RR = 3.06). Renal disease risk also tended to be high in women on hydralazine (RR = 1.95) compared with all other treatments, but this was not statistically significant and could be explained by poorer renal function and significantly higher untreated blood pressure in the hydralazine treated group at presentation. The 422 patients who were treated with methyldopa but not hydralazine had similar risk factors for cardiovascular disease compared with a group of 167 who received hydralazine but not methyldopa. Comparisons of event rates failed to find significant differences in morbidity or mortality between these two groups. The age adjusted male mortality was 14/1000 patient years on hydralazine and 12/1000 on methyldopa and 13/1000 and 6/1000 years for women respectively. There was no evidence of an increased risk of either renal disease (RR = 0.3 in men, RR = 0.3 in women) on hydralazine or weight loss (RR = 0.7 in men, RR = 1.6 in women), with similar presenting data. Systemic lupus erythematosus was a rare complication (2 of 314) of treatment with hydralazine.