This study explored the best predictors of the progress of children with autistic spectrum disorders (ASD), on some developmental domains (autistic severity, language, communication and socialisation), which are related to the core features of ASD. Eighty-six children (2.5-14 years old) with ASD, from 10 schools in Greece, were included in the study, and their developmental progress was assessed over a period of 9 months. The data indicated that, apart from the factors related to the child (e.g., non-verbal IQ, language, adaptive behaviour, autistic severity, behavioural problems at baseline, as well as the age that the child started treatment), which were correlated to the child's developmental progress, there are also factors in the child's environment that influence his or her developmental progress; the most important being the levels of parental stress, and the parents' abilities to discipline their child. These findings suggested that there are some factors (age that a child starts treatment, parental stress and parents' abilities to discipline their child) that the intervention programmes, the teachers and the parents could take into account, in order to enhance the children's gains. (Contains 8 tables.)