Background: Children with specific language impairment are known to struggle with expressive grammar. While some studies have shown successful intervention under laboratory conditions, there is a paucity of evidence for the effectiveness of grammar treatment in young children in community settings. Aim: To evaluate the effectiveness of a school-based intervention programme for expressive grammar in 5-year-olds with specific language impairment. Method & Procedures: Thirty-four 5-year-old children attending a specialized school for children with language impairment participated in the study. Nineteen children received treatment for expressive grammar (experimental group) and 15 children received a control treatment. Treatment consisted of weekly 1-h sessions of small group activities in a classroom setting for 8 weeks. Techniques included direct instruction, focused stimulation, recasting and imitation. Outcomes & Results: Results were analysed at the group level and as a case series with each child as their own control in a single-subject design. There was a significant difference in grammatical performance pre- and post-treatment for children who received grammar treatment (Cohen's "d" = 1.24), but not for a group of children who received a control treatment. Further, no difference in performance was found in the equivalent time period prior to treatment, nor for an untreated target. Treatment success was more pronounced in children without articulation difficulties which interfered with their ability to produce the grammatical targets (Cohen's "d" = 1.66). Individual analyses indicated the treatment effect was significant for the majority of children. Conclusions & Implications: Individually targeted intervention delivered in small groups in a classroom setting was effective in improving production of expressive grammatical targets in 5-year-old children with specific language impairment. (Contains 3 tables and 6 figures.)