Serum erythropoietin levels in the anaemia of chronic disorders. Serum erythropoietin (S-EPO) levels were measured in 50 patients with anaemia of chronic disorders (ACD), classified into three groups according to their aetiology: inflammatory (n = 20), infectious (n = 15) and neoplastic (n = 15). The inflammatory group showed a higher mean S-EPO level (mean value +/- SEM, 69 +/- 11 mU ml-1) than the neoplastic (43 +/- 5 mU ml-1; P less than 0.05) and infectious groups (27 +/- 4 mU ml-1; P less than 0.01). The S-EPO level in the inflammatory group also differed from that of 32 healthy controls (36 +/- 3 mU ml-1; P less than 0.05). Fourteen patients with added iron deficiency (12 subjects from the inflammatory group) showed the highest S-EPO concentration (72 +/- 17 mU ml-1). Conversely, S-EPO levels were lower in febrile subjects (12 patients with infection and five with malignancy) than in non-febrile patients (28 +/- 4 mU ml-1 vs. 55 +/- 7 mU ml-1; P less than 0.01). In the infectious group, the logarithm of S-EPO correlated directly with the haemoglobin and haematocrit values. We conclude that differences in S-EPO concentration in ACD may be further related to the patient's iron stores and temperature. A decrease in EPO production may contribute to the pathogenesis of ACD secondary to infection. 