We report a study of antimicrobial use in patients admitted to the Intensive Care Unit (ICU) in a Danish university hospital during the course of 1 year. Of 615 patients admitted, 434 (71%) received antibiotics, 220 (36%) for infections and 222 (36%) for prophylaxis. Therapy for suspected infection accounted for 67% of the total consumption of antibiotics and prophylaxis for 33%. Ampicillin was the drug most frequently used; 43% of the patients treated for infection received this drug. One hundred and thirty-one patients (60%) were treated for suspected lower respiratory tract infection. Relevant microbiological specimens were obtained from 120 (92%) of these patients and a possible pathogen was isolated in 92 patients (77%, 92/120). Staphylococcus aureus and Streptococcus pneumoniae were each isolated in almost a quarter of the patients, and Enterobacteriaceae in 53%. However, many of the pathogens isolated were of no clinical relevance and merely reflected a state of colonization. Most treatments were given during the first few days following admission. Of 220 patients receiving antibiotics for an infection, 87% were treated on day 1, but only 34% (14 of 41) on day 11. The frequent use of laboratory investigations combined with good communication between clinicians and microbiologists probably resulted in rapid cessation of unnecessary therapy. Fifty-two per cent of the antibiotics given for prophylaxis were administered later than the first postoperative day. This study emphasizes the need for consultation between surgeons and clinical microbiologists to supervise postoperative antibiotic use.