Endogenous serum thrombopoietin (TPO) levels were measured in 31 patients with aplastic anaemia (AA) using an enzyme immunoassay with a sensitivity of 20 pg/ml. The median platelet count for all AA patients was 30 plus/minus 29 X 109 /l (range 5-102) compared with a median of 284 plus/minus 59 X 109 /l (range 148-538) for normal controls. Serum TPO levels were significantly elevated in all patients compared with normals (1706 plus/minus 1114.2, range 375-5000 v 78 plus/minus 54, range 16.5-312.9, P less than 0.0001). There was no correlation between serum TPO levels and the degree of thrombocytopenia in AA patients, but TPO levels were significantly higher in patients who were platelet transfusion dependent than in patients who were transfusion independent (P less than 0.01). There was a trend for higher TPO levels in patients with severe AA compared with non-severe AA patients. Clinical trials of TPO and a related truncated, pegylated molecule, megakaryocyte growth and development factor (PEG-rHuMGDF), are awaited to determine whether treatment with these drugs will result in increased platelet counts in patients with AA.