Individuals with type 1 diabetes mellitus (T1D) at risk for Addison's disease (AD) can be identified with RIAs for autoantibodies to the adrenal antigen 21-hydroxylase (21-OHAA). Screening individuals with T1D for 21OH-AA shows a relatively high prevalence of positive autoantibodies (1.4%, 38 of 2696 subjects). After detection of 21-OHAA, individuals were evaluated with endocrine testing, including baseline cortisol, ACTH, and plasma renin activity and low (1 mug) and high (250 mug) dose cortrosyn stimulation. Typing for DR and DQ alleles and for the major histocompatibility complex class I-related chain A (MICA) gene polymorphisms was performed. Six individuals were diagnosed with AD; five were identified on initial endocrine evaluation. Follow-up over 2.9 yr yielded one additional diagnosis of AD. Endocrine testing showed a correlation between baseline ACTH and peak cortisol (r = -0.61; P < 0.0001), baseline and peak cortisol (r = 0.70; P < 0.0001), and stimulated cortisol after low- and high-dose testing (r = 0.92; P < 0.0001). DR3-DQ2/DR4-DQ8 with DRB1* 0404 was associated with expression of 21-OHAA. At 2 yr, individuals homozygous for MICA5.1 had AD-free survival of 60% compared with 100% AD-free survival in those who were not homozygous for MICA5.1. Homozygosity for MICA5.1 may increase progression to overt AD among 21-OHAA-positive individuals.