ROCKY MOUNTAIN spotted fever (RMSF) is the most important rickettsial disease of children in this country. Cases have been reported in almost all of the states of this nation proving that occurrence is not limited geographically. For this reason, it has been suggested that "tick typhus" is a more appropriate designation for the infection than is RMSF. 1 The principal pathologic lesion of RMSF, generalized capillary injury, offers an excellent explanation for the protean and often dramatic clinical manifestations of the disease and most of its complications. 1-4 The most important diagnostic sign is the rash. Although a petechial component to the rash has long been recognized, the occurrence of thrombocytopenia and other hematologic abnormalities has been noted only recently. The purpose of this report is to describe the occurrence of thrombocytopenia in RMSF. Seventeen instances of the association have been observed. The first reported case of thrombocytopenia accompanying RMSF