Concurrent with recommendations to place infants to sleep in supine, there has been a dramatic increase in the number of infants with positional plagiocephaly (PP). Recent evidence suggests that infants who have decreased exposure to prone position may have a higher incidence of PP and may be at risk for a delay in the acquisition of certain motor skills. The purpose of this study was to compare motor development between infants with PP and matched peers without PP. We also examined differences in infant positioning practices when asleep and awake between the two groups. Twenty-seven infants with PP, 3 to 8 months of age, were matched by age, gender, and race to infants without PP. Motor performance was evaluated using the Alberta Infant Motor Scale (AIMS) and the Peabody Developmental Motor Scales (PDMS). Parents completed a diary that recorded infant positioning over a 3-day period. Mean AIMS percentile score for infants with PP was 31.1 [plus or minus] 21.6 as compared with 42.7 [plus or minus] 20.2 in infants without PP (p = 0.06). Better performance on the AIMS was positively correlated with the amount of time in prone position when awake, for both groups of children (PP r = 0.52, no PP r = 0.44, p less than 0.05). Therapists should be aware of a risk of a motor delay when evaluating infants with PP. It is also important for parents to be informed about the importance of supervised prone playtime to enhance the development of early motor skills. (Contains 3 tables.)