Between September 1971 and May 1982, at the University of California at Los Angeles Medical Center, 821 total knee arthroplasties were performed in 604 patients, all of whom received perioperative antibiotics. Deep sepsis, proved by a positive culture of a specimen obtained by postoperative arthrocentesis, developed fourteen times in thirteen knees of twelve patients, an incidence of 1.71 per cent. In one of these patients, who had systemic lupus erythematosus and bilateral knee replacement, the right knee became infected with two distinct organisms on two different occasions (separated by ten months). The first infection was probably hematogenous while the second, developing after a dental procedure, definitely was. Over-all, five infections were hematogenous with an identified source and one other was suspected of having a hematogenous origin. The time from operation to the diagnosis of sepsis averaged 8.3 months over-all, but five of the fourteen infections were recognized less than two months after arthroplasty. For the six infections that were assumed to be hematogenous, the time from operation to the diagnosis of sepsis averaged 16.4 months. The major presenting symptom was pain in thirteen of the fourteen infections. The initial treatments of the fourteen infections consisted of intravenous antibiotics in all of them, primary removal of the prosthesis and so-called exchange arthroplasty after five days in one, removal of the prosthesis and fusion in one, arthrotomy and debridement in six, arthroscopic irrigation in three, and antibiotics alone in three (of which one was treated with an exchange arthroplasty after three weeks). At last follow-up, only four of the thirteen prostheses had been salvaged.(ABSTRACT TRUNCATED AT 250 WORDS)