Serum ferritin was measured in 50 patients at diagnosis of diabetes mellitus (DM) and in 20 patients with established DM and poor metabolic control. Twenty-two patients had hyperferritinemia at diagnosis. Four patients had a recognised cause for their hyperferritinemia. In the remaining 18 patients ferritin levels decreased from a mean of 506 +/- 3.6 (SE) ug/l at diagnosis to 254 +/- 29.2 ug/l seven months later (p < 0.001). Metabolic control improved significantly over the same time. All 20 patients with established DM and poor metabolic control had normal ferritin levels. When compared with the newly diagnosed hyperferritinemic patients no difference was found in levels of glycosylated haemoglobin, but ferritin values differed significantly between the two groups (p < 0.001). These results indicate that transient hyperferritinemia is a feature of newly diagnosed DM but not of established DM with poor control. If used to screen diabetic patients for haemochromatosis, serum ferritin should be measured in established DM rather than at diagnosis.