Worsening of heart failure in patients with myocardial infarction is seldom studied, elderly patients often are not included, and multivariate analysis is uncommon. The prospective PRIMA study (Prise en charge de l'Infarctus du Myocarde Aigu; management of acute myocardial infarction) sought to determine the incidence of worsening heart failure, its risk factors, and its prognostic importance in patients with myocardial infarction, regardless of age and hospital facilities, in the 'real world' in a region in France, using multivariate analysis. Data were prospectively collected in all patients with myocardial infarction admitted in all hospitals in three departments in the Rhône--Alpes region in France between 1 September 1993 and 31 January 1995. Among the 2507 patients included, 33% were in Killip classes II--IV at admission. After exclusion of patients with admission Killip class IV, 416 patients (17% of the cohort, 24% of women and 14% of men) had worsening of Killip class during the first 5 days. In-hospital mortality (overall, 14%) increased dramatically with Killip class at admission (9% in class I, 62% in class IV) and with worsening of Killip class during the first 5 days (36.5 vs. 8.5% if no worsening). In multivariate analysis, older age, diabetes mellitus and anterior Q-wave myocardial infarction were significant predictors of Killip class at admission and of its worsening; Killip class >I at admission was a significant predictor of Killip-class worsening. The significant predictors of in-hospital mortality were older age, Killip class III at admission and worsening of Killip class during the first 5 days. This large, unselected cohort revealed that, among patients with myocardial infarction, heart failure and its worsening are frequent, especially in the elderly, and dramatically worsen the in-hospital mortality.