During a 4-year study of 4,256 knee arthroscopies, eighteen patients became infected (infection rate 0.42%). Occurrence of infection was strongly associated with use of long-acting intraarticular intraoperative corticosteroids. Infection was more common among patients with longer surgery operating times, increased numbers of procedures during surgery, prior procedures, and performance of chondroplasty or soft tissue debridement. Subsequent to the study, the infection rate fell to 0.1%. Twenty-four infections were studied (our eighteen plus six other concurrent community cases); twelve were due to Staphylococcus aureus, eleven to coagulase-negative staphylococci, and one to Enterobacter cloacae. Seventy percent of the patients had onset of symptoms within 3 days of surgery. Most patients with coagulase-negative staphylococcal infections had fevers less than 38.3 degrees C (101 degrees F), negative Gram stains on synovial fluid, normal peripheral leukocyte counts, and somewhat indolent, mild clinical syndromes, while most patients with S. aureus infections had higher fevers, positive synovial Gram stains, peripheral leukocytosis, and more acute and severe clinical syndromes. Knee pain, swelling, and warmth always occurred, but erythema was noted in only 30% of patients. Treatment with 2 weeks of intravenous antibiotics was successful in all but one patient. Long-term results were excellent in sixteen of twenty-two patients.