A prospective multi-centre study was conducted to assess the microbiological pattern and prognostic factors of bacteraemia and their impact on clinical outcome. All patients admitted to 41 Italian hospitals over 2 months, from whom one or more clinically significant organisms were isolated from blood culture, were studied according to a standardized protocol and case definition. A total of 156 episodes of bacteraemia were identified in 20601 patients. There were 3·9 episodes of nosocomially acquired bacteraemia and 3·7 episodes of community-acquired bacteraemia per 1000 admissions. The most frequent pathogens isolated were Gram-negative bacteria (44·9%) but Gram-positive species accounted for 40·4% of episodes. Fungal infections due to Candida spp. were found in 3·8% of episodes, and multiple pathogens were recovered from 9·6% of episodes. The clinical response to bacteraemia was classified as sepsis in 90 episodes (57·7%), severe sepsis in 21 (13·5%) and septic shock in 26 (16·7%); 19 episodes (12·2%) showed no clinical response. The total in-hospital mortality was 25·0%. By multivariate logistic regression, the variables which independently predicted mortality were increasing age, the presence of septic shock, infection with Gram-positive bacteria or fungi and nosocomial acquisition.