Patterns and features of regression in a case series of 53 girls and women with Rett syndrome were studied at the Institute of Child Health and Great Ormond Street Children’s Hospital, London, UK. Diagnostic criteria for classical Rett syndrome were met in 46 cases, and 7 had an atypical/variant form. The most common period for regression was 12-18 months, reported by parents or in casenotes in 49% of patients. Mean age of regression was 16 months. Skills lost in order of frequency were hand use (85%), non-verbal vocalizations and simple gestures (59%), non-verbal play (51%), motor skills (49%), and words (45%). Pre-regression developmental delays were noted in more than two-thirds of cases (85% in youngest cases when parental reports were most reliable). Age at regression was not an index of neurological severity (epilepsy, breathing abnormalities, mobility, joint contractures, and oral-motor dysfunction). 
COMMENT. Signs of abnormal or delayed development (hypotonia and delayed motor milestones) are commonly observed in cases of Rett syndrome in the pre-regression period. Earliest and most frequent signs of regression are loss of hand use and communication skills. Early developmental history can aid in detection of risk factors for Rett syndrome, and before the onset of growth delay, gait ataxia, and hand stereopathies.