The interval from the time of admission to the emergency room until the administration of antibiotics in patients presenting with a serious infectious disease was analysed. Fifty patients presumptively diagnosed in the emergency room as having a serious infection (respiratory tract, urinary tract, erysipelas, fever with neutropenia or bacteremia) needing immediate empirical antibiotic treatment were enrolled in the study. A median interval from time of admission to administration of antibiotics of 5 hours was determined (range 0.6-13.3 h). The interval was significantly shorter in patients admitted at night than in patients admitted during office hours (3.7 vs. 6.0 h, P< 0.05). There was no difference with respect to the presenting features, body temperature, laboratory values at presentation or number of cultures performed. In 41 of the 50 patients blood samples were taken for culture. More than 80% of the patients received an antibiotic chosen in accordance with hospital guidelines. The analysis revealed that the median delay of 5 hours before patients received their initial dose of antibiotic depended on several factors. Attempts to provide optimal antimicrobial therapy should thus concentrate not only on the correct choice and dosage of a drug but also on prompt institution of therapy.