Childhood sleep-related rhythmic movement disorder (RMD)-sleep-related repetitive movements involving large muscle groups-can impair sleep quality, cause local injury and disturb household members. Previous parental reports indicate prevalence rates in children under 3 years of age between 5.5 and 67%. We studied the prevalence of RMD with objective home videosomnography. Methods Parents of 707 children having their one-year routine health check (357 male), 740 children having their two-year health check (395 male), and 17 children of unknown age (9 male), were asked if their child showed sleep-related rhythmic movements. If telephone interview confirmed likely RMD, parents completed a standardised clinical questionnaire and three nights of home videosomnography. Results At the one-year health check, 31/707 possible cases of RMD were identified (maximal prevalence: 4.38%; 95% CI [2.81, 5.89]) compared to 11/740 at the two-year check (maximal prevalence: 1.49%, 95% CI [0.61, 2.36]). Of 42 possible cases, 9 had resolved; 14 were uncontactable, or did not wish to participate, and 4 did not complete the study protocol. In four of ten remaining one-year olds and four of five remaining two-year olds parental report was objectively confirmed by videosomnography. Minimal prevalence based on objective observation was therefore 0.28% (95% CI [0.08, 1.30]) at one-year check and 0.41% (95% CI [0.08, 1.24]) at two-year check. Conclusions Prevalence of RMD in a large population of infants and toddlers was lower than previously reported (maximum prevalence 2.87%, minimum prevalence 0.34%). It is important to confirm parental report using objective measures. Highlights • Sleep-related rhythmic movements may have severe clinical consequences. • Prevalence was assessed in a large cohort (N=1464) of infants and toddlers. • Parental report of symptoms could be objectively confirmed in 53.3% of cases. • The prevalence in our sample is much lower (maximal 2.87%) than previously reported.