A case of familial Pelger-Huet anomaly in a 3-year-old boy with acute lymphoblastic leukaemia is described. This unique association was investigated through trial observations of the peripheral blood smear and bone marrow obtained during the child's treatment with chemotherapy. The average lobe index (ALI) of neutrophils was 42 with no three-lobed forms at the time of the initial diagnosis. During antimetabolite maintenance therapy with 6-mercaptopurine and methotrexate the ALI was 1.87 and three-lobed forms were present. The behaviour of the P-HA cells to heat induced radial hypersegmentation of the nucleus was examined in other family members. The mechanism by which heat and folate deficiency induce neutrophil segmentation is preserved in the familial Pelger-Huet anomaly.