The late infantile regression period of Rett syndrome was analysed in a consecutively collected series of females, born 1945-87 (median 17 1/2 years), fulfilling the diagnostic criteria for classic Rett syndrome. For general information, data from 91 girls and women were used (group A), while more detailed analysis was based on the youngest 20, all born during the last decade (group B). Median ages at onset of developmental stagnation (stage I) were 11 (5-24) and 10 (5-18) months for groups A and B, respectively. Loss of acquired skills (stage II) began at 19 (12-36) and 17 (13-25) months, and lasted for 19 (2-53) and 13 (2-32) months. Onset of the period of loss was distinctly marked in 43%, dramatic in 16% and insidious in 41%. Gross motor delay and aberrant behaviour raised the first suspicions of disease, while a more specific pattern comprising a triad of manifestations, i.e. contact/communication, hand use/skill, babble/words, characterized the loss of acquired skills. Although development invariability came to a definite break at a crucial stage of maturation, an "awakening" (stage III) and return of interest to act and interact astonishingly followed the period of withdrawal. Apraxia was found in all, but some use of the hands could be enticed in engaging and non-demanding situations. The observed preference for use of the left hand (9/11), compared to the right (2/11), suggested a reflection of an underlying dysfunction/disease process, also displayed as spike activity in left central leads in EEGs from 13/20 girls.