We evaluated clinical and microbiological aspects in 145 hospitalizations of 89 intravenous drug abusers with acute soft tissue infections at the injection site. There were 58 superficial abscesses, 27 deep abscesses, 57 cellulitis with or without concomitant ulcer, 1 purulent arthritis, 1 tenosynovitis and 1 incompletely categorized abscess. The commonest location was the groin. Serious complications occurred in 17 cases, including 4 lower-extremity amputations due to arterial lesions. There was a predominance of polybacterial infections (53 percent polybacterial, 38 percent monobacterial, 9 percent sterile). The commonest bacteria isolates were Streptococcus species with a preponderance of oropharyngeal bacteria, Staphylococcus aureus, and anaerobes, especially Bacteroides species. Typical intestinal bacteria were rare. In addition to surgical treatment we recommend that aerobic and anaerobic culturing with susceptibility tests always are carried out, that primary antibacterial therapy should consist of an antistaphylococcal agent, such as dicloxacillin plus metronidazole, and that free injection paraphernalia with disinfection swabs are easily available.