OBJECTIVE: To test 4-year-olds, using 14 maneuvers of the Childhood Myositis Assessment Scale (CMAS), comparing healthy children with those with juvenile dermatomyositis (DM). METHODS: Healthy 4-year-olds (n = 28) completed the CMAS. Their scores were compared with children with juvenile DM (n = 18) who had a muscle Disease Activity Score (DAS-M) of 0. RESULTS: The healthy children achieved a mean +/- SD CMAS score of 46.6 +/- 2.3 (interquartile range 46-47). There were no significant differences between boys and girls, and the scores were not significantly associated with height or weight. The greatest variation involved items that assessed endurance. Item 1, neck raise, yielded a mean +/- SD score of 28.2 +/- 19.3 seconds, with a mean +/- SD CMAS score of 2.5 +/- 0.9 (maximum score 5). Item 3, leg lift, yielded a mean +/- SD score of 55.5 +/- 37.3 seconds, with a mean +/- SD CMAS score of 3.1 +/- 1.1 (maximum score 5). Item 5, sit-ups maneuver, yielded a mean +/- SD score of 5.3 +/- 1.1 sit-ups. Almost identical data were obtained for the 18 treated children with juvenile DM who had normal strength on the DAS-M. CONCLUSION: Healthy children ages 4 years do not achieve the total CMAS score of 52 attained by older children. Both boys and girls were remarkably consistent, with a mean CMAS score of 46.6. Children ages 4 years with juvenile DM with a DAS-M of 0 also achieved a CMAS score of 46.6. We conclude that half of 4-year-old children achieve a mean CMAS score of 46 or 47, not a total CMAS score of 52, suggesting that weakness may be overdiagnosed in 4-year-olds with an inflammatory myopathy.