In 1982 and 1983 practicing pediatricians in a Lyme disease-endemic county, reported 90 cases of Lyme disease among children 19 years of age and younger (median age, 9 years). Three-fourths of the children had initial symptom onset in the summer months, with peak incidence in July. Infection occurred twice as often in boys than in girls, and tick bites were recalled by less than half (49%) of the children or parents. Erythema chronicum migrans was present in two-thirds (67%) of the cases with median onset 7 days after a definite tick bite. Arthritis or arthralgia occurred in 59% and neurologic symptoms, especially seventh nerve palsy, occurred in 14%. Asymmetric involvement of a few large joints, especially the knee, was most commonly reported for those with joint involvement. antibiotics were prescribed for 79% of the children, three-fourths of whom were treated with oral penicillin. Initial diagnosis of Lyme disease is usually made on clinical grounds alone because serologic tests are often negative. Serologic tests for antibody to Borrelia burgdorferi were more often positive in cases with neurological or joint involvement, in addition to erythema chronicums migrans (80%), than in cases presenting with erythema chronicums migrans only.