It was investigated if IGF-1 levels in cats which experience diabetic remission (i.e. transient diabetes mellitus) differ from those in cats with permanent disease. Thirteen of 32 diabetic cats showed remission within 16 weeks after initiating insulin therapy, 19 cats continued to need insulin therapy. IGF-1 concentrations were measured before (t(0)), 1-3 (t(1)) and 4-8 (t(2)) weeks after initiating insulin therapy. No difference in IGF-1 levels was found between cats with transient and permanent diabetes at any point in time. In both groups of cats IGF-1 concentrations were significantly lower compared to those of controls before insulin administration. After starting insulin therapy IGF-1 increased significantly in both groups. In cats with transient diabetes IGF-1 levels were not different from controls already at t(1), whereas in cats with permanent diabetes it took until t(2). Although IGF-1 levels seem to normalize faster in cats with transient diabetes mellitus, measurement is not helpful to predict the course of the disease.