Skin biopsies were performed to examine dermal capillary basement membranes for albumin by immunofluorescence microscopy in children and young adults with Type 1 (insulin-dependent) diabetes from simplex and multiplex families, their non-diabetic parents and control subjects. Circumferential vessel staining for albumin was 12.0 +/- 14.4% (mean +/- SD) in diabetic patients from multiplex families, 18.8 +/- 20.0% in diabetic patients from simplex families, and 18.5 +/- 24.2% in unaffected parents from simplex families. These values were significantly higher (p less than 0.005) than those obtained in unaffected parents from multiplex families (0.4 +/- 0.9%) or control subjects (0.8 +/- 1.7%). There was no statistically significant difference between Type 1 diabetic probands from simplex or multiplex families and unaffected parents from simplex families. In the simplex families, positive capillary staining for albumin was present only in parents from families in which the diabetic proband had positive staining. In the parents, the presence of dermal capillary staining for albumin did not correlate with an abnormal glucose tolerance test or the presence of cytoplasmic islet cell antibodies. These observations suggest an abnormality in the dermal capillary basement membrane in some unaffected parents of Type 1 diabetic patients from simplex families.