In a pediatric case of severe multiple-site osteomyelitis caused by Borrelia burgdorferi, the presence of spirochetes in a bone lesion was documented both by culture and by the polymerase chain reaction (PCR). Positive PCR results were also obtained with culture fluid yielding spirochetal growth and with acute-phase serum. Although the disease evidently was a late manifestation of Lyme borreliosis, antibodies to B. burgdorferi were low in titer and were restricted to the IgM class. The distribution of osteomyelitic lesions in multiple bones and the positive PCR results obtained with serum argue for hematogenous spread of the spirochetes. Before the specific diagnosis was established, the patient received several potent antimicrobial drugs, without a favorable outcome. In contrast, therapy with ceftriaxone led to a rapid cure that persisted thereafter. We conclude that infection due to B. burgdorferi must be considered a possible cause of subacute pediatric osteomyelitis.