The pathogenesis of anaemia associated with human immunodeficiency virus infection is still far from being understood. It cannot be explained by direct effects of the virus on the haematopoietic system. Recent data suggest a role for immune activation. In a cross-sectional study we compared blood cell counts, haemoglobin and erythropoietin levels of 63 HIV-seropositive individuals with immune activation markers (interferon-gamma, serum and urine neopterin, and beta 2-microglobulin) and with parameters or iron metabolism (serum iron, transferrin, free iron binding capacity, ferritin). We found significant correlations between the concentrations of haemoglobin and the immune activation markers and erythropoietin concentrations. Additional significant correlations existed between the parameters of iron metabolism and haemoglobin levels, and ferritin correlated inversely with transferrin. In sum, low haemoglobin levels in patients were associated with enhanced cellular immune activation, as seen by increased interferon-gamma, neopterin and beta 2-microglobulin, and with changes of iron metabolism: low haemoglobin was associated with low transferrin and free iron binding capacity and high ferritin levels. Endogenous release of cytokines such as interferon-gamma-inhibiting erythropoiesis may be one underlying cause of anaemia in these patients.