In a randomised, placebo-controlled, double-blind clinical study the clinical efficacy and safety of Crataegus extract WS 1442, standardised to 18.75% oligomeric procyanidines, were investigated in 40 female and male outpatients suffering from congestive heart failure NYHA class II. Following a wash-out period of up to seven days, the patients were randomised to be treated for 12 weeks with either WS 1442 (3 x 1 capsule) or placebo. The primary outcome variable was exercise tolerance determined with bicycle exercise testing; as a secondary outcome variable the difference of the double product was calculated. On average, the exercise tolerance increased by 66.3 W x min (10.8%) in the WS 1442 group while in the placebo group a reduction of 105.3 W x min (16.9%) was measured. This difference between the groups was borderline statistically significant (p = 0.06). During the three month therapy the difference of the double product (heart rate x systolic blood pressure x 10(-2)) decreased by 14.4 mmHg s(-1) (26.8%) in the WS 1442 group and by 1.3 mmHg s(-1) (2.7%) in the placebo group, respectively. Recording of laboratory parameters and adverse events showed that WS 1442 was safe and well tolerated. The data show that Crataegus extract WS 1442 is clinically effective in patients with congestive heart failure corresponding to NYHA class II.