Cushing syndrome due to primary adrenocortical nodular dysplasia was diagnosed in two patients, aged 3 years 9 months and 9.5 years. Subsequently, adrenalectomy was performed and followed by steroid replacement. In both cases, the adrenals were normal or only slightly enlarged and showed adrenocortical nodular dysplasia histologically. Small lymphocytic infiltrates consisting of T-cells and class II MHC positive macrophages were present in adrenal specimens of both the patients. Samples of protein A sepharose purified serum immunoglobulins from both children stimulated adrenocortical DNA synthesis and cortisol production in cultured guinea-pig adrenal segments in vitro in a dose dependent fashion. Adrenal stimulating immunoglobulins were also demonstrated in serum specimens of both patients' mothers. However, none of them had overt signs of adrenal disease. Our data support the view that autoimmune mechanisms may be involved in primary adrenocortical nodular dysplasia.