Neutrophil hypersegmentation (NH) is an important haematological feature of cobalamin or folate deficiency. As iron deficiency and folate deficiency often occur in the same target groups it is important to establish whether iron deficiency alone is a cause of NH. We report a case-control study which addresses this issue. Two groups of hospital patients were studied. Group 1 comprised 50 patients with iron deficiency anaemia (IDA). Group II comprised 50 control age- and sex-matched patients who were haematologically normal without evidence of iron deficiency from the iron studies. Patients with other factors which could affect the degree of neutrophil segmentation (cobalamin/folate deficiency, renal failure, infection and drug exposures) were excluded from the study. A total of 10 000 neutrophils were examined, 100 from each patient. NH was defined as the presence of five or more five-lobed neutrophils per 100, or any neutrophils with six or more lobes. The results were as follows: IDA, mean neutrophil lobe count 3.36; number of patients with NH 31/50 (62%): controls, mean neutrophil lobe count 2.96, number of patients with NH 2/50 (4%). These differences were statistically significant. We conclude that NH is common in IDA. The mechanism whereby iron deficiency results in NH is not clear.