SUMMARY.Using a platelet counting technique, spontaneous platelet aggregation (SPA) was determined in citrated whole blood (WB) and platelet-rich plasma (PRP) obtained from 27 patients with insulin-dependent diabetes mellitus and from 32 healthy controls. SPA in WB, but not in PRP, was significantly enhanced in the patients compared with the controls. In the patient group, SPA in WB correlated positively with poor metabolic control as measured by glycosylated haemoglobin (Hb A1). Furthermore, the increased SPA in WB from diabetics was found to be associated with an increased mechanical fragility of red blood cells (RBC) measured as liberation of haemoglobin into the plasma in stirred samples of WB. Pentoxifylline, which is believed to increase the deformability of RBC, reduced the extent of SPA in WB from the patients but was without effect on SPA in PRP from the patients or in WB from the controls.