Tick-borne rickettsiae of the genus Ehrlichia have recently been recognized as a cause of human illness in the United States. In the years 1986-1988, 10 cases of ehrlichiosis were diagnosed in children in Oklahoma. Fever and headache were universal: myalgias, nausea, vomiting, and anorexia were also common. Rash was observed in six patients but was a prominent finding in only one. Leukopenia, lymphopenia, and thrombocytopenia were common laboratory abnormalities. Six patients were treated with tetracycline, three with chloramphenicol, and one was not treated with antibiotics: all recovered. The onset of illness in spring and early summer for most cases paralleled the time when Amblyomma americanum and Dermacentor variabilis are most active, suggesting that one or both ticks may be vectors of human ehrlichiosis in Oklahoma.