Chronic idiopathic neutropenia (CIN) is a disorder characterized by severe neutropenia and a maturational arrest of the neutrophil precursors in the bone marrow. The pathogenesis of this disorder has been obscure. We examined the production of endogenous G-CSF and the expression of G-CSFmRNA in a patient with adult type CIN. The G-CSF production by the patient's mononuclear cells was deficient in spite of the adequate accumulation of G-CSFmRNA. Our data suggests that the defect in the endogenous G-CSF production at the post-transcriptional level is likely to be an aetiological factor in CIN.