Fifteen diabetics with recurrent painful local reactions to insulin were studied. Reactions occurring after intradermal insulin injection were observed in nine patients over 48 hr and biopsies were taken at intervals for microscopic study using the 1 mu Giemsa technique. Insulin-specific IgE and IgG levels were measured on all patients. Five patients had biphasic reactions in which wheal and flare (WFR) were followed by an indurated lesion 4 to 6 hr later. These reactions lasted up to 24 hr and were histopathologically identical to similar "late-phase reactions" seen with ragweed. They were transferable with Prausnitz-Küstner (P-K) testing. Three patients had reactions that developed 8 to 12 hr after injection, peaked around 24 hr, and were not preceded by WFR. These reactions were morphologically delayed hypersensitivity reactions and were not transferred by P-K testing. One patient had a reaction that developed in 4 to 6 hr after injection and peaked by 12 hr. Histologically, this reaction was "Arthus" in type and was not transferred by P-K testing. Specific insulin antibody determinations were not helpful in distinguishing patients with different types of reactivity. These data show that recurrent local reactions to insulin may be of three distinct types: "late-phase reactions" (which are IgE dependent), "Arthus" local vasculitic reactions, or tuberculin-type delayed hypersensitivity reactions. These findings may influence the approach to management of these reactions.