Patients from a renal transplantation unit with an unusually high incidence of polycythaemia were divided into polycythaemic and control groups. The rate of rise of haemoglobin concentration was not significantly different in the two groups. The polycythaemic group received a significantly lower dose of azathioprine (p < 0.005) and included more patients with polycystic disease than the control group (p < 0.05). An effect of azathioprine on bone marrow function was suggested by the polycythaemic group also having a higher mean white cell count (p < 0.02). Azathioprine dosage correlated negatively with post-transplantation polycythaemia regardless of the original cause of renal failure.