The aim of the study was to test the hypothesis that raised platelet-activating factor (PAF) may contribute to the aetiology of developmental dyslexia. PAF is a potent proinflammatory mediator which signals cell damage and facilitates natural killer cell activity. Raised PAF may help protect against tumourigenesis. As dyslexia has a partial genetic basis, the PAF hypothesis predicts that dyslexia may be negatively associated with a family history of cancer. To test this prediction, children with dyslexia (n=163) and children without dyslexia (n=154), with (n=152) and without (n=165) a family history of cancer (total n=317; mean age 11 years 5 months, SD 2 years 11 months), were compared on standard psychometrics (British Ability Scales subtests). Results showed that proportionately fewer children with dyslexia (38%) than controls (58.4%) had a family history of cancer, and there was some evidence of a ‘dose’ effect: children who had more relatives with cancer showed better reading and spelling. It was concluded that children at genetic risk of dyslexia who have a family history of cancer have better reading and spelling than those without a family history of cancer, confirming the prediction of the PAF hypothesis.