We have measured the specific binding of 125I-insulin to insulin receptors on circulating monocytes from 5 patients with insulinomas. In all patients the concentration of insulin receptors was inversely related to the basal, circulating level of insulin (corrected for proinsulin content). In the four patients with chronically elevated concentrations of plasma insulin, receptor concentrations were decreased. In three of these patients there were also marked alterations in receptor affinity. With insulinomas, correlations with the clinical state appear to be clearer when receptor concentration and especially receptor affinity are considered in addition to circulating insulin and proinsulin levels.