The goal of this study was to analyse changes in classroom interaction quality in 85 early childhood classrooms in vulnerable social contexts in Santiago de Chile which participated in the first year of a professional development programme. Classroom interaction quality was measured using the Classroom Assessment Scoring System (CLASS) at the beginning and end of the school year. Contingency tables were built in the experimental and control groups and subsequently analysed with the chi-square and odds ratio. Results show that most classrooms maintained low quality levels, even after having received the intervention. A significant decrease in quality level of emotional support was also observed in the control group. These results have important implications for quality evaluation policy and practices and for the design and implementation of professional development programmes in early childhood education. [Translation from Spanish by Mary Black.]