The blood pressures of 79 consecutive patients with myotonic dystrophy have been shown to be significantly lower than those of a control series. Reanalysis of previously published data on a group of 17 myotonic dystrophy patients shows a similar result. The possibility that relative hypotension may confer a selective genetic advantage on asymptomatic gene carriers in the community is considered. It is suggested that, since patients with minimal clinical evidence of disease are also hypotensive, measurement of blood pressure may be useful as an adjunct to other methods of preclinical diagnosis of myotonic dystrophy.