Purpose: This study investigated the effects of a complex sentence treatment at 2 dosage levels on language performance of 30 school-age children ages 10-14 years with specific language impairment. Method: Three types of complex sentences (adverbial, object complement, relative) were taught in sequence in once or twice weekly dosage conditions. Outcome measures included sentence probes administered at baseline, treatment, and posttreatment phases and comparisons of pre-post performance on oral and written language tests and tasks. Relationships between pretest variables and treatment outcomes were also explored. Results: Treatment was effective at improving performance on the sentence probes for the majority of participants; however, results differed by sentence type, with the largest effect sizes for adverbial and relative clauses. Significant and clinically meaningful pre-post treatment gains were found on a comprehensive oral language test, but not on reading and writing measures. There was no treatment advantage for the higher dosage group. Several significant correlations indicated a relationship between lower pretest scores and higher outcome measures. Conclusions: Results suggest that a focused intervention can produce improvements in complex sentence productions of older school children with language impairment. Future research should explore ways to maximize gains and extend impact to natural language contexts.