This study measures physical activity levels in a representative population-based sample of older adults (aged [greater than or equal to]50 years) with intellectual disabilities. For this, the steps/day of all 1050 participants of the Healthy Ageing and Intellectual Disabilities study (HA-ID; a study conducted among three Dutch healthcare providers in 2009-2010), were measured with a pedometer. Largely due to physical limitations (n = 103), walking speed less than 3.2 km/h (n = 252), limited understanding or non-cooperation (n = 233), only 257 of the group were able to participate in valid measurements with pedometers. Of these 257 participants, only 16.7% (95% CI 12.2-21.3) complied with the guideline of 10,000 steps/day, 36.2% (95% CI 30.3-42.1) took 7500 steps/day or more, and 39% (95% CI 32.6-44.5) was sedentary (less than 5000 steps/day). Because the measured sample was the more functionally able part of the total sample, this result is likely to be a considerable overestimation of the actual physical activity levels in this population. This realistic study shows that physical activity levels are extremely low in adults aged 50 years and over with intellectual disabilities. Focus on lifetime promotion of physical activity in this specific, but rapidly growing population, is recommended. (Contains 4 tables and 1 figure.)