Between January 1992 and June 1995, 443 patients underwent surgical treatment for infections of the hand and upper extremity at Cook County Hospital. Most admissions were for human bites (51%). Other causes included cellulitis (17.0%), septic arthritis (11.8%), abscess formation (9.9%), gangrene (2.7%), flexor tenosynovitis (2.5%), osteomyelitis (1.8%), dog bites (1.4%), web space infections (0.9%), paronychia (0.45%), and cat bites (0.23%). Bacterial cultures were obtained in 395 cases; sensitivities were obtained in 247 of the cultures. Cultures grew a total of 719 organisms. By far, the most common organisms isolated were Gram-positive aerobes (61.9%), including streptococcal species (29.5%), Staphylococcus aureus (15.3%), and coagulase-negative Staphylococcus (11.5%). Most of the organisms isolated were common flora of the mouth or skin. Cultures reflected an increased incidence of Gram-negative enteric and anaerobic organisms compared with similar studies in the literature. Bacterial sensitivities reflected increased organism virulence compared with similar studies in the literature. Bacteria were resistant to the first-line antibiotic therapy of penicillin G (2-4 million units every 4-6 hours) and cefazolin (1 g every 8 hours) in 16.2% of cultures. The predominant resistant organism was Staphylococcus aureus, which grew resistant strains in 19.6% of cultures tested. Enterococcus grew resistant strains to the first-line regimen in 47%. The Enterobacteriaceae grew resistant strains to the first-line regimen in 66.6%. Acinetobacter grew resistant strains to the first-line regimen in 92%. Pseudomonas grew resistant strains to the first-line regimen in 100%. The addition of gentamicin (5 mg/kg every morning) in cases of suspected intravenous drug abuse did not significantly alter the results. Sensitivity results were also analyzed to determine the effectiveness of specific antibiotics against the prevalent resistant organisms. Vancomycin (1 g every 12 hours) proved most effective against the Gram-positive organisms. Ciprofloxacin (400 mg every 12 hours) proved most effective against the Gram-negative organisms, including Pseudomonas, and was also the most effective single agent overall.