OBJECTIVE--To measure the prevalence and incidence of intermittent claudication, to describe the mortality associated wtih the condition, and to assess the relevance of risk factors for vascular disease. DESIGN--A standard questionnaire on calf pain when walking was given in the prospective Speedwell study, and a range of risk factors were measured. The men were re-examined at intervals of three years, and deaths over 11 years were identified. SETTING--The general population. PARTICIPANTS--All men aged 45 to 59 registered with 16 general practitioners. RESULTS--The prevalence of intermittent claudication increased from almost nil at ages 45-49 to 2.9% at ages 60-64. The annual incidence increased from 0.3% in the youngest men to 0.5% in those in their early 60s. Intermittent claudication was related to the existence of ischaemic heart disease, particularly angina, at the first examination. The relative odds of men with angina developing intermittent claudication was 6.7 (95% confidence interval (95% CI) 3.6 to 12.4). The risk of death in men with intermittent claudication was substantially raised. After standardisation for age and smoking the relative odds of death was 3.8 (95% CI 2.2 to 6.5). The excess was entirely from circulatory causes. Systolic blood pressure, fasting plasma glucose, triglycerides, and white cell count were all independently associated with the development of intermittent claudication, but the most striking association was with smoking. CONCLUSIONS--Intermittent claudication is an indicator for a very high risk of death. This is only partly explained by its strong association with ischaemic heart disease.