A patient with acute megakaryoblastic leukaemia is described in whom exactly the same paracentric inversion of 3q was detected as in three previously documented cases. The patient's serum thrombopoietin (TSF) was significantly raised. Based on these findings we postulate a role for a gene (? oncogene) on chromosome 3q in thrombopoietin production. Abnormalities of 3q may assist in delineating a subgroup of acute nonlymphocytic leukaemia, namely acute megakaryoblastic leukaemia.