This study provides an analysis of the diagnostic trends in autistic spectrum disorder (ASD) for children aged under 17 years in the Rhondda and Taff Ely districts of South Wales. In the period 1988-2004, 336 children received a diagnosis of ASD and represent the case registry data of one community pediatric team. For the period 1994-2003, the caseload of 267 comprised Asperger (58%), Kanner (20%) and "other forms" of autism (22%). In comparison to the previous 5 year period, diagnosis of ASD in 1999-2003 increased for Asperger (2.9 fold) and "other forms" (4.4) but decreased for Kanner autism (0.69). Male:female gender ratios and age at diagnosis fell for all subcategories of ASD. Prevalences per 10,000 children born in Rhondda Taff Ely are ASD 61.2, Asperger 35.4, Kanner autism 12.7, and "other forms" 13.0. Detected prevalences and trends for ASD are in line with national standards and do not show an increase for Kanner autism. (Contains 1 figure and 2 tables.)