BACKGROUND Sirolimus (SRL) has been implicated in the causation of post-transplantation anaemia (PTA). It also induces profound red blood cell (RBC) microcytosis, which is poorly understood. METHODS We conducted a retrospective study of SRL-induced anaemia and microcytosis [mean corpuscular volume (MCV) <80 fl] with specific reference to iron homeostasis in 93 renal transplant patients treated with SRL for at least 3 months. RESULTS While mean haemoglobin (Hb) and use of erythropoiesis-stimulating agents increased on SRL, RBC MCV underwent a significant decline throughout the whole study period of 24 months (P < 0.001) with the percentage of microcytosis rising from 2.2% at the start of SRL therapy to 40.7% after 24 months of therapy. An association between dMCV (MCV change on SRL) and SRL levels was shown at 3, 6, 12 and 24 months post-SRL (P = 0.015, P = 0.037, P = 0.002 and P = 0.001, respectively). Intravenous (IV) iron administration was an independent predictor of dMCV at 12 and 24 months on SRL (P = 0.031 and P = 0.048, respectively). All patients who, after starting SRL and seeing a fall in MCV, then went on to receive IV iron therapy, showed a marked increase in MCV; this did not happen to patients given oral iron therapy. CONCLUSIONS SRL is associated with mild anaemia, but marked RBC microcytosis-these phenomena are correlated with SRL levels and the use of IV iron. Functional iron deficiency and impaired gastrointestinal absorption of iron seem likely explanations.