The results of treating 235 hypertensive patients who had been prescribed nifedipine in a hypertension clinic were examined for factors affecting blood pressure response and the frequency of side-effects. Pretreatment systolic and diastolic blood pressure correlated significantly with the decrease in blood pressure but this effect was lost following statistical correction. No relation was found between response and age or race nor did any biochemical or haematological parameter predict the antihypertensive effect. Fifty-nine (25%) patients complained of side-effects which were dose related; the drug had to be discontinued in 30 patients (13%) but the remaining 29 continued at the same or reduced dosage. Small, but statistically significant, elevations were seen in serum albumin, alkaline phosphatase and bilirubin as well as a rise in average blood glucose levels. Although side-effects are fairly common nifedipine is an effective antihypertensive drug when given alone or in combination with other therapies.