The incidence of various types of arthritis in children was estimated by a prospective 1-year study in the greater Helsinki area (population under 16 years of age: 148,362). Patients were sought from primary care physicians, and 71% of the patients studied were seen within 1 week of the onset of symptoms. All patients received followup care for at least 3 months; patients whose symptoms were prolonged received followup care for a minimum of 2 years. The incidence per 100,000 children under 16 years of age was as follows: 108.5 for all cases of arthritis, 6.7 for septic arthritis, 5.4 for enteropathic arthritis, 51.9 for transient synovitis of the hip, 18.9 for prolonged arthritis (duration greater than 3 months), and 25.8 for acute transient arthritis. The incidence of juvenile rheumatoid arthritis was 19.6. Oligoarticular disease was prevalent (76%) among the juvenile rheumatoid arthritis patients.