Cohen syndrome is an autosomal recessive disorder characterized by mental retardation, microcephalia and typical craniofacial features, myopia and chorioretinal dystrophy. As some patients were reported to have leucopenia, we collected the haematological data of 26 Finnish Cohen patients. They all had experienced periods of isolated granulocytopenia from an early age. Granulocytopenia was mild to moderate, non-cyclic and never fatal. Most patients suffered from prolonged or repeated gingival or skin infections. We restudied 16 patients. Bone marrow examination revealed in all patients a normo- or hypercellular marrow, with a left-shifted granulopoiesis in 8/16 patients. The response to adrenaline stimulation was subnormal in 12/14 and to hydrocortisone in 8/16 patients, but administration of rhG-CSF caused granulocytosis in the three patients studied. No bone marrow malignancies were seen.