The HLA antigen frequencies in 100 Caucasian patients with adrenocortical hyperfunction were compared with those found in 352 healthy unrelated subjects. Fourteen antigens on the HLA--A locus, seventeen antigens on the HLA--B locus and three antigens on the HLA--C locus were determined using the standard NIH microlymphocytotoxicity test. The frequency of HLA--A1 antigen in the patient group was 49% as compared with 28% in the controls (pcorr less than 0.01). An increased frequency of HLA--B8 and HLA-BW35 antigens was also found, but the difference was not significant. Increased A1--B8 and A1--BW35 haplotype frequencies were observed. The relationship between the HLA system and various endogenous and exogenous factors eliciting hypercorticism, together with complementary family studies indicate that the HLA system may be a useful genetic marker of the disease susceptibility gene.