A retrospective open study was conducted to determine the efficacy of penicillin and ceftriaxone in children with skin manifestations of Lyme borreliosis (solitary erythema migrans, multiple erythemata, borrelial lymphocytoma) and neuroborreliosis, respectively. One hundred sixty children were treated with penicillin and 41 with ceftriaxone for an average of 12 days. Serum antibodies to borreliae were determined before therapy and 2-3 and 4-6 weeks thereafter. At admission 44%/26%,8%/42%, and 40%/35% of erythema migrans, borrelial lymphocytoma and neuroborreliosis patients, respectively, were IgM/IgG positive. Four to 6 weeks after treatment the percentage of seropositives was 20%/15%,8%/61%, and 21%/44%, respectively. A 3 months follow-up was completed with 151 children. No child showed clinical evidence of illness, nor were there abnormalities in laboratory parameters.