Aim: This is a cross-national study with the aim to explore the development of children with autism over time in the UK and Greece. The focus of the study was to investigate the differences in language and social skills between children with autism across the two countries who were receiving different types of treatment: speech and language therapy, psychoanalytic/psychodynamic psychotherapy, and occupational therapy. Study design: A cross-national longitudinal design with a mixed (between-subjects and within-subjects) design. Participants: A sample of 40 children in total. In the UK, 20 children with autism who had received psychotherapy (n = 10) and speech and language therapy (n = 10) were recruited and monitored post-therapy twice over a two-year period. In Greece, 20 children with autism who received occupational therapy (n = 10) and speech and language therapy (n = 10) were recruited and monitored post-therapy twice over a two-year period. Results: All children changed significantly over time on all aspects of measurement, demonstrating that children with autism are developing in a very similar way across the two countries. With respect to the effect of the therapy context on the development of children with autism, it was found that there were no differences across intervention contexts at the start of the study, and there were mainly nonsignificant interactions in the rate of change across the differing types of intervention. However, further analysis showed some important differences: speech and language therapy participants presented more widespread change on language scores across the measures; psychotherapy participants showed significant greater increase in imagination and decrease in stereotypical behavior; and occupational therapy participants presented significant reduction of stereotypical behavior. Conclusions: This study can help professionals who work with children with autism further their understanding of the disorder and how it manifests through time in order to provide appropriate services based on each child's needs.