In the early 1950s, the blood pressure of 3901 Dutch civil servants and their spouses aged 40-65 years was measured in a general health survey. Isolated systolic hypertension (systolic pressure greater than 160 mmHg, diastolic pressure less than 90 mmHg) was observed in 6.3% of the women and 3.0% of the men. The prevalence increased with age and it was more common in women in all age groups. Using logistic regression, with adjustment for potential confounders (age, smoking, serum cholesterol, Quetelet index, alcohol consumption, haemoglobin level, pulse rate and diastolic blood pressure) the association of 15- and 25-year total mortality with isolated systolic hypertension was determined. Compared to normotensive people (systolic pressure less than or equal to 135 mmHg, diastolic pressure less than 90 mmHg), the risk of death from all causes was significantly higher for men with isolated systolic hypertension after 15 and 25 years of follow-up (odds ratio OR = 2.4, 95% confidence interval (CI) 1.2-4.8 and OR = 3.2, 95% CI 1.3-8.0). For women 15-years mortality risk was strongly associated with isolated systolic hypertension (OR = 3.7, 95% CI 1.4-9.7). The increased risk was less pronounced after 25 years of follow-up (OR = 1.7, 95% CI 0.96-3.0). Our results support those of other studies and indicate that isolated systolic hypertension is an important independent risk factor for all-cause mortality. Since isolated systolic hypertension may be an indicator for the early onset of ageing, it is important to study its determinants and to pay more attention to its diagnosis and treatment in middle-aged populations.