Products of the pro-opiomelanocortin gene have been shown to have marked effects on insulin secretion. Selective antagonists of beta-endorphin have been reported to correct the impaired insulin secretory response to glucose seen in non-insulin dependent diabetes. Families with an autosomal dominant pattern of inheritance of diabetes were studied for possible linkage between diabetes and the pro-opiomelanocortin locus by examining the inheritance of a Rsa 1 restriction fragment length polymorphism. In two families with classical Type 2 diabetes there were recombinants between the disease and the pro-opiomelanocortin locus. In a family with maturity-onset diabetes of the young there were only two informative meoises, but there was a crossover between the disease and the pro-opiomelanocortin gene locus. Inherited defects in or near the pro-opiomelanocortin gene locus are unlikely to be directly involved in the aetiology of non-insulin dependent diabetes.