The development of complications does not depend entirely on diabetes duration and control. Red-blood-cell Na/K-ATPase plays a central role in the regulation of intra- and extracellular cation homeostasis. Alteration of this transport enzyme is thought to be linked to several complications of diabetes mellitus. The aim of this study was to find out any association between diabetic complications and red-blood-cell Na/K-ATPase activities in type 2 diabetes mellitus. Sixty-seven patients and 25 controls were enrolled in the study. Patients were evaluated for retinopathy, neuropathy, and nephropathy. The membrane Na/K-ATPase activities were measured. The studies were done twice with and without ouabain. The results of the calculations are written as micromol Pi/mg protein/h. The duration of diabetes and enzyme levels were negatively correlated (r = -0.38, p = 0.001). Na/K-ATPase enzyme activity was significantly lower in the diabetic patients than the control group (p < 0.0001). In neuropathic patients the activity was also significantly lower (p < 0.0001). The enzyme activities of the people with retinopathy were significantly lower than the ones without retinopathy (p < 0.001). The enzymatic activities did not differ among the degrees of nephropathy. The results indicate that erythrocyte Na/K-ATPase enzyme activities are decreased in type 2 diabetes and the decrement of the enzyme is correlated with the diabetes duration.