Dyspnoea and syncope are common causes of admission to hospitals. Pulmonary embolism is often a differential diagnosis, and by examining the clinical history the clinician searches for known predisposing factors. This case report highlights the importance of Klinefelter's syndrome as a predisposing factor for venous thromboembolism. The syndrome is caused by an extra X chromosome in men, among whom the prevalence is estimated to be 1:500-1:1000. Probably only 25 % of men with the syndrome are diagnosed. A man in his forties was admitted to hospital due to dyspnoea and syncope. CT showed submassive pulmonary embolism. The course illustrates the challenges of pulmonary embolism and its association with Klinefelter's syndrome. Several studies have shown an increased incidence of venous thromboembolism in patients with Klinefelter's syndrome. Klinefelter's patients have a higher pre-test likelihood of venous thromboembolism than other patients, similar to patients with hereditary thrombophilia. Klinefelter's syndrome is a persistent risk factor for recurrent thromboembolism. Thus, Klinefelter's syndrome impacts both the diagnosis and treatment of thromboembolic disease.