Deformational plagiocephaly, a non-synostotic asymmetry of the occipital bone, is a frequent occurrence in infants, with a peak incidence of 19.7% at 4 months of age. One aetiology is hypothesized to be due to restriction in normal cervical spine motion.This study aimed to determine if an association exists between plagiocephaly and restrictions in the passive range of motion (PROM) of the cervical spine.A retrospective analysis of the clinical records of 150 consecutive cases was performed, with specific inclusion/exclusion criteria applied. Data were collated and analysed.Private chiropractic practice, Victoria, Australia.Participants were 150 infants under 12 months of age, with a mean age of 122 ± 60.6 days and a range in age of 42 to 245 days.The outcome included measurement of the cervical PROM, identification of the location of the restriction, and a decision about the presence of plagiocephaly.In the 150 cases, 78.7% were found to have restrictions in cervical PROM, with 60.2% showing an indication of plagiocephaly. In the cases with plagiocephaly, 92.2% were found to have restrictions in cervical PROM.The presence of plagiocephaly was associated with a higher prevalence of restriction in cervical PROM. A statistically significant association existed between restriction of the occipitoatlantal joint and development of contralateral deformational plagiocephaly. Future clinical trials assessing the effectiveness of spinal manipulative therapy in the treatment and management of restricted cervical PROM and plagiocephaly in infants are needed.