The aim of this study was to estimate the possibility of predicting the course of type 1 diabetes. We analyzed the importance of islet cell antibody levels and residual beta cell function in 46 newly diagnosed patients with diabetes. Islet cell antibodies (ICAs; Juvenile Diabetes Foundation [JDF] units) were determined at the time of diagnosis by the indirect immunofluorescent method. beta cell function was estimated by C peptide levels (nmol/L) before and after glucagon stimulation at the time of clinical remission. Of the 46 patients, 13 were ICA negative (group A). Among ICA-positive patients, ICAs were < 20 JDF units (group B) in 15, between 20 and 80 JDF in 9 (group C), and > 80 JDF in 9 (group D). In group A, 9 patients had clinical remission for 7.5 +/- 1.7 months. Their basal C peptide level was 0.26 +/- 0.05 nmol/L and it increased after stimulation to 44.5 +/- 2.5%. Ten patients in group B had remission for 6.2 +/- 1.5 months. Their basal C peptide levels (0.28 +/- 0.07 nmol/L) were similarly increased after stimulation (47.5 +/- 2.5%). In group C, all patients had remission and it was of the longest duration (14.7 +/- 1.5 months). They had the highest basal C peptide levels (0.45 +/- 0.12 nmol/L) with increases to 57.5 +/- 3.5%. Seven patients in group D with ICA levels > 80 JDF had a short remission (3.2 +/- 1.2 months) despite good basal C peptide levels (0.42 +/- 0.05 nmol/L) and excellent increases after stimulation (92.5%). Our results suggest that moderate levels of ICA are associated with good residual beta cell function and longer remission. Very high ICA levels (> 80 JDF) at the time of diagnosis despite better beta cell function are associated with short clinical remission. Therefore, high ICA levels could be a marker of strong autoimmune reaction and accelerated depletion of beta cell function.