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OBJECTIVE: To evaluate clinical, laboratory, and radiological features and outcomes in patients with monoarthritis (MA), identified in a cohort of patients with early arthritis. METHODS: A cohort of 270 patients with undiagnosed arthritis of less than 1 year's duration was divided into 3 groups: single episode of MA (MA, n = 27), MA with a history of patient-reported arthritis (MA + past, n = 23), and oligo- or polyarthritis (OA/PA, n = 220). At 6-month intervals, all patients underwent a standardized examination, radiographs, and standard laboratory tests including rheumatoid factors (RF), antiperinuclear factor (APF), antikeratin antibody (AKA), anticyclic citrullinated peptide antibody (anti-CCP), antinuclear antibodies, and HLA-AB-DR typing. After a median followup of 30 months, the diagnosis was evaluated by a hospital-based rheumatologist. RESULTS: Age and sex did not differ across the 3 groups. Knee involvement was more common in the MA group than in the MA + past group (p < 0.03), whereas hand and metatarsophalangeal involvement was less common (p < 0.03 and p < 0.0001, respectively). RF and anti-CCP were less often positive in the MA group than in the MA + past group (p < 0.02 and p < 0.001, respectively) and the OA/PA group (p < 0.02 and p < 0.03). No patient in the MA group received a diagnosis of rheumatoid arthritis (RA). RA was less common and disease modifying antirheumatic drugs were prescribed less often in the MA group than in the other 2 groups (p < 0.0001 for both comparisons). CONCLUSION: The MA group was clearly different from the other groups, with a favorable outcome and no risk of progression to RA.