The presence of large numbers of moles, atypical in appearance and distribution typifies the atypical mole syndrome. The syndrome may occur in one individual alone or in an autosomal-dominant fashion in his/her family. The presence of this phenotype indicates an increased risk of melanoma, although the risk varies according to the presence or absence of a family history. The classification devised by Kraemer et al. works well in estimating the risk of melanomas but is, as yet, not fully evaluated. The diagnosis of the syndrome is essentially clinical, necessitating the consideration of various aspects of phenotype such as total mole count, distribution, clinical appearance of the moles, the age of onset, site, and number of melanomas. It is always helpful to screen first-degree and second-degree relatives if there is a family history of moleyness or melanoma. Small numbers of clinically and, sometimes histologically dysplastic naevi may occur in normal individuals. The vast majority of these probably regress as do totally banal naevi, although a low percentage will result in a melanoma. Those patients should be questioned to establish the existence of a family history of increased numbers of moles and melanoma. They should also be thoroughly examined for the presence of other signs of the syndrome: increased total mole count, iris freckles etc. In the absence of either a family history or these additional clinical features it is unlikely that these individuals have 'the syndrome'. Common sense advice about sun avoidance and self-examination should be given. It is important to distinguish this from the atypical mole syndrome (AMS).(ABSTRACT TRUNCATED AT 250 WORDS)