Background: Research into early identifying features and associated symptoms of autism has shown high prevalence rates of motor skill deficits in children with autism spectrum disorders (ASD). This research has shown evidence of deficiencies in a wide range of motor skills in children with autism, but there is little agreement in the field on whether motor skill impairments are a defining feature of autism or not. This study explored differences in motor development on parental reports of motor skill at 18 months and 36 months, and individual differences in motor development among 3-4 year old children on a standardized measure of fine motor skills (Mullen Scales of Early Learning, fine motor section, or MSEL). The design of the study included comparisons between the ASD group, children with mental retardation without autism (MR), and typically developing children (TD). The relationship between levels of cognitive functioning (IQ) and fine motor skills was also explored. The current study is based on data from the Norwegian Mor & Barn Study (MoBa) and the Autism Birth Cohort study (ABC study). Methods: The present study is divided into two sub-studies based on: 1) parental reports of motor skills at 18 and 36 months and 2) the MSEL fine motor section and IQ based on the Stanford-Binet V. The results from the parental reports in sub-study 1 were compared between the ASD and TD group at each point in time. The results from the measures in sub-study 2 were compared between the HF-ASD, TD, LF-ASD and MR groups. The relationship between IQ and fine motor skills was also examined closely. Results: Mean scores on both parental reports of motor skill and a standardized measure of fine motor skill (MSEL) were significantly lower for children with autism than typically developing children. Mean scores on the MSEL were not different between the ASD group and the MR group. Levels of cognitive functioning were strongly associated with performance on the MSEL across groups. Mean standardized scores on parental reports of motor skill were not significantly different at 18 months and 36 months, indicating that children with delays in motor development at 18 months continue to be delayed at 36 months as well. Conclusion: Children with ASDs are a highly varied group, but as a whole they are more likely to have deficits and/or delays in motor development compared to typically developing children of the same age. The variation in motor development appears strongly associated with intellectual functioning, but the difference in fine motor skill between the typically developing children and the children with ASD continues to be significant after the effects of IQ have been controlled for. These findings are discussed in relation to methodological factors, interventions and causal theories.