The Hedgehog signalling pathway is important in embryological development and is highly conserved through evolution. Recently Patched, a member of the pathway, was found to be important in Gorlin's syndrome. Inherited Patched gene mutations underlie the syndrome, in which a key feature is multiple basal cell carcinomas (BCCs). The gene is also mutated in sporadic BCCs as well as in sporadic occurrences of other tumours seen in Gorlin's syndrome. The precise mechanism whereby Patched gene mutation leads to tumour development is not known, but BCC is characterized by relentless local invasion and only rarely metastasizes. This suggests that abnormalities of the Hedgehog pathway account for these features. This proposal is discussed in the context of what is already known about the normal function of the Hedgehog pathway and its deregulation in cancer. Copyright © 2000 John Wiley & Sons, Ltd.