In 16 patients (9 on azathioprine, 7 not) the ineffective iron turnover (I IT) was much higher in the azathioprine group (62.7 ± 6.7 vs. 23.5 ± 3.5 μmol/l blood/ day, p < 0.0001, 2-tailed t test), though the red cell iron turnover (ROT) was similar (42.8 ± 2.9 vs. 41 ± 4.8). Erythropoietin improved the anaemia in all patients and raised the RCIT (4 still on azathioprine to 72.2 ± 9.8, p < 0.003; 7 non-azathioprine patients to 62.7 ± 5.3, p < 0.01); the ΠT remained higher in the azathioprine-treated (85.5 ± 19.3 vs. 37.1 ± 5.4; p < 0.013). In 2 patients who discontinued azathioprine, the IIT declined markedly to normal. In summary, azathioprine exacerbates the anaemia of renal failure by augmenting ineffective erythropoiesis, while erythropoietin benefits those on azathioprine as much as other renal patients by stimulating both effective and ineffective eryth