BACKGROUND: To analyze the serial changes in cranial asymmetry (CA) during helmet therapy for deformational plagiocephaly (DP). METHODS: The subjects were 159 patients with DP who visited National Center for Child Health and Development between October 2011 and March 2014 and completed helmet therapy. The authors retrospectively collected information from medical records and analyzed the rate of improvement of deformation. RESULTS: The average age at the start of the helmet was 24.1 weeks old [standard deviation (SD): 5.0], and the average treatment period was 21.2 weeks (SD: 5.3). The average CA before the start of helmet therapy was 16.3 mm (SD: 4.2), and it improved to 7.7 mm (SD: 3.5) after therapy. The analysis in the present article was limited to 16 weeks after initiation of helmet therapy when 1/4 of patients completed the therapy. In general, improvement of CA occurred inverse proportionally to the enlargement of head circumference. In subgroup analysis, for those who started earlier than 24 weeks of age or those with the severity of Argenta 1 or 2, improvement occurred rapidly and reached nearly a plateau at 12 weeks of treatment. For those started later or those with more severe deformity, improvement occurred slower and it continued even after 12 weeks of treatment. CONCLUSION: The rate of improvement was rapid at the early stage of treatment for those started earlier or with milder severity. This rapid improvement was obtained when the cranial enlargement was rapid. By understanding the course of improvement, evaluation of treatment responsiveness and ideal timing to finish therapy is possible.