An adrenal tumour was incidentally discovered with no clinical signs of Cushing's syndrome. The endocrine evaluation revealed the unique hormonal constellation of an increased urinary cortisol excretion rate, unequivocal suppressibility of plasma and urinary cortisol by dexamethasone, but only to a residual level in the low normal range which probably reflected ACTH-independent 'autonomous' cortisol secretion. After removal of the adrenal mass, urinary cortisol secretion and dexamethasone suppressibility were normalized. In vitro, the tumour cells were as sensitive towards ACTH as 'normal' human adrenal cells, but showed a reduced cortisol production rate per cell. We suppose that the adrenal mass participated in the diurnal rhythm of ACTH-mediated cortisol secretion in vivo, which resulted in an increased cortisol secretion. During the night, when ACTH levels were low, the cortisol production decreased and the hormone levels were probably too low to suppress ACTH. We regard the hormonal findings in our patients as 'Pre-Cushing's syndrome', although the absence of clinical features of Cushing's syndrome remains unclear. We suggest that every patient with an incidentally discovered adrenal mass should have an endocrinological evaluation because the results may help to decide whether or not the adrenal tumour should be removed.