Immunocompetent antigen-presenting Langerhans cells were investigated in skin biopsies of 20 short-term Type 1 (insulin-dependent) diabetic patients and compared with 17 matched normal control subjects. Langerhans cells in epidermal sheet preparations were visualized with a monoclonal anti-HLA DR antibody using indirect immunofluorescence. A significant decrease of Langerhans cells/mm2 body surface area was found in 10 patients immediately at the onset of diabetes compared to 10 patients with 6 months duration of diabetes and to normal control subjects (401 +/- 30 vs 559 +/- 43 vs 611 +/- 33, p less than 0.01 and p less than 0.002). There was no significant difference in the number of Langerhans cells between patients with 6 months duration of diabetes and control subjects. Examination of the most likely precursor of Langerhans cells, the blood monocytes, indicated an increase of monocyte counts in Type 1 diabetic patients after 6 months duration (344 +/- 37 cells/microliters vs 191 +/- 31 in control subjects, p less than 0.05) and an inverse correlation between the number of Langerhans cells in skin with the number of monocytes in peripheral blood (at onset: r = -0.73, p less than 0.01, after 6 months of diabetes: r = -0.61, p less than 0.05). In addition, a positive correlation between Langerhans cells and daily insulin dose was noted in patients after 6 months of diabetes (r = 0.76, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)