The purpose of the study was to count and characterize the range of stereotypies--repetitive rhythmical, apparently purposeless movements--in developmentally impaired children with and without autism, and to determine whether some types are more prevalent and diagnostically useful in children with autism. We described each motor stereotypy recorded during 15 minutes of archived videos of standardized play sessions in 277 children (209 males, 68 females; mean age 4y 6mo [SD 1y 5mo], range 2y 11mo-8y 1mo), 129 with autistic disorder (DSM-III-R), and 148 cognitively-matched non-autistic developmentally disordered (NADD) comparison children divided into developmental language disorder and non-autism, low IQ (NALIQ) sub-groups. The parts of the body involved and characteristics of all stereotypies were scored blind to diagnosis. More children with autism had stereotypies than the NADD comparison children. Autism and, to a lesser degree, nonverbal IQ (NVIQ) <80, especially in females contributed independently to the occurrence, number, and variety of stereotypies, with non-autistic children without cognitive impairment having the least number of stereotypies and children with autism and low NVIQ the most. Autism contributed independently to gait and hand/finger stereotypies and NVIQ <80 to head/trunk stereotypies. Atypical gazing at fingers and objects was rare but virtually limited to autism. Stereotypies are environmentally modulated movement disorders, some highly suggestive, but not pathognomonic, of autism. Their underlying brain basis and genetic correlates need investigation.