Normal left ventricular systolic performance with impaired left ventricular diastolic filling may be present in a substantial number of patients with congestive heart failure (CHF). To evaluate the effect of oral verapamil in this subset, 20 men (mean age 68 +/- 5 years) with CHF, intact left ventricular function (ejection fraction greater than 45%) and abnormal diastolic filling (peak filling rate less than 2.5 end-diastolic volumes per second [edv/s]) were studied in a placebo-controlled, double-blind 5-week crossover trial. All patients underwent echocardiography to rule out significant valvular disease, and thallium-201 stress scintigraphy to exclude major active ischemia. Compared to baseline values, verapamil significantly improved exercise capacity by 33% (13.9 +/- 4.3 vs 10.7 +/- 3.4 minutes at baseline) and peak filling rate by 30% (2.29 +/- 0.54 vs 1.85 +/- 0.45 edv/s at baseline) (all p less than 0.05). Placebo values were 12.3 +/- 4.0 minutes and 2.16 +/- 0.48 edv/s, respectively (difference not significant for both). Improvement from baseline in an objective clinico-radiographic heart failure score (scale 0 to 13) was significantly greater with verapamil compared to placebo (median improvement in score: 3 vs 1, p less than 0.01). Mean ejection fraction and systolic blood pressure were unchanged from baseline; diastolic blood pressure and heart rate decreased to a small degree. Verapamil may have therapeutic efficacy in patients with CHF, preserved systolic function and impaired diastolic filling.