This study was designed to assess the overall prevalence of infection among the patients in hospitals in the UK and the Republic of Ireland. Patient data were collected and entered directly into a portable Olivetti (A12 notebook) computer with a custom-designed program (Epi-Info version 5.01). The statistical analysis was performed using the Statistical Package for Social Sciences software (SPSS). In all, 37,111 patients from 157 centers were studied, and a mean hospital acquired infection (HAI) prevalence rate of 9.0% (range 2-29%) was calculated. HAI rates were higher in teaching hospitals (11.2%) than in non-teaching hospitals (8.4%) P < 0.001. Four major sites of infections--infections of the urinary tract (23.2%), surgical-wound infections (10.7%), lower-respiratory tract (22.9%) and skin infections (9.6%)--accounted for 66.5% (2559 of 3848) of the total infections identified.