We studied the binding and action of insulin in cultured fibroblasts from six patients with lipoatrophic diabetes and marked in vivo insulin resistance and from seven control subjects. The binding of insulin was not altered, which corresponds well with studies with circulating erythrocytes. Similarly, the action of the hormone on amino acid uptake (estimated by active transport of aminoisobutyric acid) was comparable in patient and control cells. Conversely, studies concerning the effect of insulin on glucose transport (estimated by facilitated diffusion of 2-deoxyglucose) or glycogen synthesis (estimated by incorporation of glucose into cellular glycogen) revealed the presence of heterogeneous alterations among the different patient cell lines. However, although the nature of the defect(s) varied among the patients, alterations in glucose metabolism were present in all cases. These data suggest the presence of primary postbinding defects in glucose cellular pathways that give rise to insulin resistance in cells from lipoatrophic diabetic patients.