Background: Continued professional development (CPD) is required for updated skills and knowledge. This study evaluates the efficacy of a CPD programme for mainstream school teachers. Aims: In an 11-week intervention programme, speech-language therapists (SLTs) presented the participating teachers with whole-class teaching techniques aimed at creating a language and communication-supporting classroom environment. The effects of the intervention on the language development of the students in the teachers' classes were assessed. Methods & Procedures: A total of 211 first- and second-year students (M[subscript age] = 7;6, range = 6;5-8;9) underwent standardized language assessments of receptive and expressive language abilities before and after their teachers' participation in the CPD. The students were divided into intervention and delayed intervention groups to enable randomized intervention allocation. Linear mixed modelling was used to estimate the individual and interaction effects of group, time and demographic factors. Outcomes & Results: Significant effects of time, group and school, respectively, but no interaction between time and group indicates that while all students advanced between assessments, the progress was not attributable to the teachers' participation in the CPD. Conclusions & Implications: Results are discussed in light of those of recent studies of universal services to support optimal language development.