Background: Literature on the prevalence of hypertension in people with intellectual disability (ID) is mostly based on file studies or on measurements limited to the age group below 50 years. We measured and calculated the prevalence of hypertension in adults with ID and studied the distribution of hypertension in relation to age, gender, diagnosis of Down's syndrome and level of ID. Methods: In an observational cross-sectional study, standardized blood pressure measurements were obtained from 258 randomly selected adult clients of three Dutch care providers for people with ID. Hypertension was defined as a mean systolic blood pressure above 140 mm Hg on repeated measurements. Results: The overall prevalence rate of hypertension was 17.4% (95% CI 12.28-22.46). This was comparable to the prevalence in the general Dutch population. No accurate blood pressure measurement could be performed in 28.4% of people with severe and profound ID owing to repetitive physical activity or resistance. Hypertension was significantly related to older age and absence of Down's syndrome; no correlation with gender or level of ID could be shown. Recommendation: The risk factor hypertension should be detected and treated in the same manner as in the general population following national guidelines.