The aim of the study was to examine the clinical outcome of patients presenting to an early arthritis clinic with synovitis of the knee. The patients were assessed at presentation for evidence and pattern of joint inflammation. These patients were then reassessed at 3, 6 and 12 months and thereafter annually to determine clinical outcome. One thousand six hundred and thirty-three consecutive referrals were examined, 903 of whom had early synovitis. One hundred and thirty had knee synovitis at presentation, of whom 73 fulfilled ACR criteria for rheumatoid arthritis (RA) during the study. All 73 presented with a symmetrical polyarthritis that included the small joints and had persistent disease at 1 year. Of the remaining 57 patients, 61% of those presenting with an oligoarthritis and 33% with a polyarthritis (including knee synovitis) were in remission at 1 year. None of those presenting as a monoarthritis of the knee had inflammation at 1 year or fulfilled ACR criteria for RA at any time. It was concluded that patients presenting with knee synovitis in the absence of a small joint polyarthritis usually have a benign course following standard therapy. No patient who presented with monoarthritis developed RA. Knee synovitis as part of a polyarthritis (even when not fulfilling ACR criteria) probably justifies disease-modifying antirheumatic drug at presentation.