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Objective. The purpose of this study was to examine clinical severity, multidimensional development, and adaptive behavioral functioning in younger and older children with Rett syndrome (RTT) in the pseudostationary stage (stage III). Methods. Fourteen younger ([less than or equal to]10 years of age) and 15 older (11-18 years of age) children with confirmed stage III RTT (assigned to young-RTT and old-RTT groups, respectively) participated in this study. Clinical severity was determined using the Clinical Severity Score (CSS) scale for RTT. The children's cognitive, language, motor, and sociocommunicative development was assessed using the Mullen Scales of Early Learning (MSEL) and the Early Social Communication Scale (ESCS). Their adaptive behavioral and daily functional skills were assessed using the Vineland Adaptive Behavior Scales-Chinese version (VABS-C) and Pediatric Evaluation of Disability Inventory-Chinese version (PEDI-C). Results. Compared with the young-RTT group, the old-RTT group had higher severity of scoliosis on the CSS scale, poorer fine motor scores on the MSEL, reduced eye contact, reduced alternating eye gaze, and reduced turn-taking during social interaction on the ESCS. However, none of the VABS-C or PEDI-C subscale scores differed significantly between the groups. Higher CSSs were significantly correlated with lower scores in several subscales of MSEL, ESCS, VABS-C, and PEDI-C, especially for gross motor, mobility, and socialization functioning in all children with RTT. Conclusion. Age-related differences in fine motor and sociocommunicative skills were observed between the young-RTT and old-RTT group, as measured using standardized assessments. Greater severity of RTT was correlated with poor motor, sociocommunicative, adaptive behavioral, and daily functional skills in stage III RTT. Impact. Practitioners should be aware of clinical severity and the differences of developmental and adaptive behavioral functioning between younger and older children in the pseudostationary stage of RTT to provide specific age-related treatments. Lay Summary. With an understanding of severity and differences of developmental and adaptive behavioral functioning between younger and older children, clinical professionals can provide specific age-related treatments. Keywords: Children, Development, Pseudostationary, Rett Syndrome, Severity, Introduction Rett syndrome (RTT; MIM 312750) is an X-linked progressive neurodevelopmental disorder that is commonly linked to methyl-CpG-binding protein 2 (MECP2) mutations and mainly affects girls. (1,2) It is characterized [...]