This paper presents a retrospective review of 6 cases of severe neutropenia attributed to suramin, the response to granulocyte-colony stimulating factor (G-CSF) and the possible mechanism. Plasma suramin concentrations, G-CSF, platelet-derived growth factor-AB (PDGF-AB) and fibroblast growth factor basic (FGF basic) levels were measured and correlated with neutropenic course. The time course of neutropenia was unpredictable and occurred both during and following discontinuation of suramin. Neutropenia rapidly resolved with G-CSF. Neither the measured growth factor levels nor plasma suramin concentrations correlated with neutropenia. We conclude that neutropenia secondary to suramin is unpredictable and responds to G-CSF administration permitting further suramin therapy. The mechanism remains unknown.