Adrenal medullitis in type I diabetes. To investigate whether cell-mediated immunity against the adrenal medulla occurs in type I diabetes (IDDM), we conducted a retrospective autopsy study of adrenal glands from IDDM and nondiabetic subjects using formalin-fixed tissue. Forty-four IDDM subjects, aged 4-67 yrs (mean +/- SD, 44.8 +/- 15.4) with a duration of IDDM from 0-55 yr (28.6 +/- 14.2), and 29 nondiabetic controls, aged 8-82 yr (51.8 +/- 18.6), were evaluated for a lymphocytic infiltrate using UCHL1, which recognizes a subpopulation of resting T-lymphocytes and most activated T-lymphocytes. Immunohistochemistry using antihuman B-cell antibody (L26) was also performed. Sections were scored for both lymphocytic infiltrates and fibrosis [none (0), small (1), moderate (2), or large (3)]. Blinded scoring was performed. A moderate to severe UCHL1 infiltrate was present in 9 of 44 (20%) IDDM, compared with 1 of 29 (3%) control subjects (P less than 0.04). Mild to severe fibrosis (score 1, 2, or 3) was present in 22 of 42 (52%) IDDM subjects compared with 4 of 25 (16%) control subjects (P = 0.003). Eight of 42 (19%) IDDM subjects had moderate to severe fibrosis (score 2 or 3) compared with 1 of 25 (4%) control subjects. Seventeen of 44 (39%) IDDM subjects had either a moderate to large cellular infiltrate or moderate to severe adrenal medullary fibrosis compared with 2 of 29 (7%) control subjects (P = 0.003). Staining of the adrenal medulla with L26 revealed a large cellular infiltrate in only one subject who was UCHL1 negative. Adrenal medullitis was observed in 20% of IDDM subjects, suggesting that the adrenal medulla may be another immunological target in IDDM. 