Angiotensin converting enzyme inhibitors (ACEI) are established drugs for the treatment of congestive heart failure. Cases of symptomatic hypotension, especially on the first day of treatment, have been reported occasionally. The database we analysed consisted of 1,177 patients, mean age approximately 70 yrs, with congestive heart failure NYHA functional class II or III. These patients were treated and observed prospectively according to a uniform protocol, starting therapy with 2.5 mg enalapril and measuring blood pressure at hourly intervals for eight hours thereafter. 94.6% of the patients experienced no symptomatic hypotension, 4.75% moderate symptoms (e.g. dizziness, headache) and 0.59% severe symptoms (e.g. fainting, collapse, renal failure). For the analyses of risk factors a large number of baseline variables were analysed univariately to select those significant for inclusion in a multivariate stepwise logistic regression. Alternatively the CART-(classification and regression tree) technique was used. Both techniques showed diastolic blood pressure < or = 70 mmHg to be the single most significant risk factor. CART-analyses showed also pretreatment with nitrates and systolic blood pressure < or = 120 mmHg to be of prognostic relevance. Thus CART is a valuable complement when looking for prognostic factors.