Background: Most standardized intelligence tests require more than 1hour for administration, which is problematic when evaluating individuals with intellectual disabilities and developmental disabilities (IDDD), because a significant proportion of these individuals can not tolerate lengthy evaluations. Furthermore, most standardized intelligence tests are of limited usefulness for individuals with severe cognitive deficits because of floor effects. Methods: A number of low-difficulty items were selected from standardized tests. A total of 271 participants with profound, severe, moderate and mild levels of cognitive impairment took part in this study. In the formative phase, 68 participants were evaluated with the selected items, and those items that differentiated between levels of cognitive impairment were retained in the battery. The instrument was then modified and standardized with an additional 203 participants. Results: The instrument, referred to as the Rapid Assessment for Developmental Disabilities (RADD), required 10-25 min for administration. Internal reliability estimates from the RADD total score and from individual subtests satisfied conventional and rigorous statistical criteria (median alpha r = 0.93). The RADD total score was strongly correlated with the level of cognitive impairment (rho = 0.86). The RADD total score and individual subtests differentiated between all levels of cognitive impairment (Wilks [logical conjuction] = 0.135, F[subscript 42,525.832] = 12.075, P less than 0.001). Receiver operating characteristic curves indicated the instrument was particularly sensitive to the cognitive abilities of the most seriously impaired participants. Conclusions: The RADD, composed of low-difficulty items from published tests, is rapidly administered, assesses a wide range of cognitive skills and differentiates among all levels of cognitive impairment. The battery has clinical utility with populations exhibiting short attention spans because of its ability to quickly assess a wide range of cognitive abilities. The RADD also has research potential for the documentation of cognitive function in studies of individuals with IDDD.