About one woman in 1,000 has an extra X chromosome, but such women have no recognised characteristic somatic features and little is known about their long-term health and cancer risks. We conducted a cohort study of mortality and cancer incidence in 542 women diagnosed with X polysomy at 25 cytogenetic centres in Britain since 1959. Fifty-nine deaths occurred during follow-up to mid-2004. Mortality was significantly raised (standardised mortality ratio (SMR) = 2.5 (95% confidence interval (CI) 1.9-3.2)), with excess deaths due particularly to cardiovascular disease (SMR = 2.5 (95% CI 1.5-3.8)) and respiratory disease (SMR = 4.0 (95% CI 1.7-7.9)). Risks of cancer incidence and cancer mortality overall were not raised, but there was significantly raised mortality from non-Hodgkin's lymphoma (NHL) (SMR = 10.4 (95% CI 1.3-37.6); based on 2 cases). The data indicate that mortality in women diagnosed with X polysomy is considerably raised. The raised risk of NHL is seen also in males with more than one X chromosome, and hence although unexpected and based on small numbers, it might indicate the action of a gene on the X chromosome, possibly in the pseudoautosomal region, that escapes X-inactivation.