A cohort of children with Lyme arthritis was used to evaluate the clinical and serologic profile of the disease. During a 42-month period (June 1989 to December 1991), 44 patients (13 girls and 31 boys, ages 4-18 years) were included and followed for 6-36 months. Inclusion required the presence of arthritis, as well as positive serology. Thirty-four children with juvenile rheumatoid arthritis or spondyloarthropathy were used as a serologic comparison group. Five different patterns of arthritis were found. Preceding erythema migrans was seen in seven children. Antinuclear antibodies were positive in 30% of the patients. Three treatments were used and selected according to physician preference, patient age, and presence of extraarticular disease: amoxicillin, doxycycline, and ceftriaxone. Articular disease reached complete resolution in all patients within 2-12 weeks. Lyme arthritis in children may mimic other pediatric arthritides. Prognosis for children with clearly defined Lyme arthritis was excellent.