Two cases of children with essential thrombocythemia (ET) with the presence of a Philadelphia chromosome (Ph1) are presented and discussed. Diagnosis was based on their clinical presentation and marked primary thrombocytosis. The site of the Ph1 translocation, as detected by a 1.2-kb bcr genomic probe, differed in the two patients. These cases, along with other reported cases of Ph1-positive ET in the literature, suggest that the presence of the Ph1 cannot be used to rule out a diagnosis of ET. Additionally, the differing translocation sites in these cases suggests that the exact translocation site may not be significant in determining which cell lineage will predominate in a Ph1-positive myeloproliferative disorder.