Throughout the Asia-Pacific region, despite government efforts to promote the use of information and communication technologies (ICT) in schools, the acceptance and use of these technologies by classroom teachers has been slow to take off. This study focused on ascertaining the effectiveness of a school-based, on-site, and ongoing professional development program conducted in a primary school in Hong Kong. There were training sessions conducted by fellow teachers, and participatory action research groups to share resources and experience in incorporating ICT in teaching. Teachers’ incorporation of computers in teaching was significantly increased in the first year of intervention but showed fluctuation in the second year. A number of explanations for these changes were offered by teachers in their interviews. The lower self-ratings in year two may be due to changes in self-perception rather than changes in actual ability or practices.