Plant thorn synovitis (PTS) is an uncommon cause of monoarthritis. Seven cases of PTS were identified at our institution from January 1979 to July 1990, six of whom were men. Mean age was 27 years (range, 7 to 56 years). Symptoms included pain, swelling, and stiffness. Synovitis was present on examination along with decreased range of motion of affected joints in all patients. Roentgenograms were unremarkable in five patients, but disclosed demineralization in two others. Initial conservative treatment with nonsteroidal antiinflammatory drugs (NSAIDs), antibiotics, or splinting was usually unsuccessful; surgery was necessary in six patients. Findings included marked inflammatory synovial reactions with evidence of retained thorn in all patients. One patient had a positive operative wound culture (Enterobacter agglomerans) without evidence of osteomyelitis. All patients improved after surgery without sequelae. Despite a history suggesting thorn injury in many cases, diagnosis was often delayed; mean time to diagnosis was 10 weeks (range, 2 weeks to 9 months). PTS must be included in the differential diagnosis of monoarthritis. Histologically, PTS can mimic sarcoidosis, tuberculosis, or fungal infection. Optimal treatment of PTS is arthrotomy, foreign body removal, and extensive synovectomy.