Acromegaly is usually regarded as a disease which arises sporadically except on those rare occasions in which it occurs as one component of an inherited pluriglandular syndrome. Familial acromegaly occurring in isolation has been reported only rarely. We have studied three families in each of which two first-degree relatives have developed acromegaly without clinical evidence of other endocrinopathies, in order to clarify the relationship of familial acromegaly to multiple endocrine neoplasia, type 1 (MEN 1). No further subclinical cases of acromegaly have been detected in those first-degree relatives of the index cases who were screened. In two of the families, we have found no evidence of abnormal pancreatic or parathyroid function either in the acromegalic index cases or their asymptomatic relatives: a diagnosis of MEN 1 is therefore effectively excluded. In the third family, the presence of multiple subcutaneous lipomata in several members of the family and elevation of PTH levels in two (normocalcaemic) acromegalic index cases, might indicate a highly atypical presentation of MEN 1. None of the six acromegalic index cases had elevations of circulating growth hormone releasing hormone levels. We conclude that isolated familial acromegaly may be more common than has hitherto been realized and that, in these families at least, ectopic production of growth hormone releasing hormone is not responsible.