Aberrant receptors have been implicated in the pathogenesis of several types of adrenal tumours. So far the presence of aberrant receptors has been investigated in patients with massively enlarged adrenals due to ACTH-independent macronodular adrenal hyperplasia (AIMAH) and unilateral adrenal adenomas associated with overt or subclinical Cushing's syndrome. The likelihood of aberrant responses in patients presenting with bilateral adrenal incidentalomas (BI) presenting as discrete solitary bilateral adenomas has not been thoroughly addressed. This is an observational cross-sectional prospective study conducted in a secondary/tertiary care centre. We studied 33 patients; 28 with incidentally discovered bilateral discrete adrenal adenomas and five with massive bilateral adrenal macronodular hyperplasia. We probed responses to physiological stimuli, namely upright posture and meal; in a subgroup of menopausal women the presence of aberrant gonadotropin receptors was assessed by the LHRH test. Abnormal responses obtained in tests performed with no dexamethasone suppression were always repeated and confirmed under dexamethasone suppression. Aberrant cortisol responses were confirmed in 10 patients; 9 to posture, 1 to meal (along with a positive response to posture) and 1 to LHRH tests. Patients who responded to any test compared to those who tested negative had larger adenomas, higher post-LDDST and midnight cortisol and a trend for lower ACTH levels. Patients without subclinical hypercortisolism (SH) did not respond to any test while 50% of patients with SH had an aberrant response (P = 0.002). A greater prevalence of aberrant responses was noted in patients with bilateral macronodular hyperplasia compared to those with solitary bilateral adenomas (80 vs. 21.4%, P = 0.02). Aberrant cortisol responses, primarily to posture testing, are present in a substantial proportion of patients with bilateral adrenal incidentalomas. Such cortisol responses are observed only in patients with subclinical hypercortisolism and especially in those patients with larger adrenal lesions.