The study was undertaken to analyse whether the presence or the induction of TNF-alpha, a potent inhibitor of haemopoiesis, might affect the clinical response to treatment with interleukin-3 in patients with myelodysplastic syndromes. A total of 15 patients were treated with IL-3. Baseline serum TNF-alpha levels were elevated in MDS patients (14.2 +/- 2.4 pg/ml) compared to healthy controls (9.1 +/- 1.1 pg/ml). During IL-3 therapy TNF-alpha levels remained unchanged in 3/14 patients in whom platelet counts increased, while in non-responders TNF-alpha levels increased 1.9-fold (P < 0.025). These findings indicate that TNF-alpha not only is induced during IL-3 therapy in MDS patients but that this elevation might be associated with a poor platelet response to therapy.