In a placebo-controlled trial 62 patients with chronic congestive heart failure (CHF) (New York Heart Association class III) had hydralazine (149 +/- 11 mg daily) or placebo added to conventional therapy. During 12 months' follow-up 27 patients dropped out, 15 of 32 in the hydralazine group and 12 of 30 among the control subjects. The 1-year mortality rate was 28% in the hydralazine group compared to 27% in the control group. Symptomatic improvement was noted in both groups; however, it was gradually more pronounced in the actively treated group with a statistically significant difference between the two groups at month 12 (p less than 0.05). The hydralazine patients increased their exercise capacity 25%, from 53 +/- 3 watts at month 0 to 67 +/- 4 watts at month 12 (p less than 0.01). No improvement in exercise capacity took place in the placebo group. A significant improvement in chest x-ray examination was found with hydralazine (p less than 0.01) in contrast to a significant deterioration among the control subjects (p less than 0.05). Thus, we conclude that hydralazine used in chronic CHF has beneficial clinical effects during long-term treatment.