Lexicon Pirate was originally developed as a strategy intervention programme to treat lexical disorders of pre-school children. To evaluate the therapy's effectiveness for school-age students, a randomized controlled trial (RCT, N = 157) was conducted. Based on a pre--post-test design, the programme's impacts were compared with a control group (CG) that did not receive the strategy training. Potential long-term impacts were analysed with a follow-up test (four months after the intervention was completed). Therapeutic success is interpreted by an improved performance in standardized tests compared to the CG. The experimental group (EG) made significant to highly significant progress on both lexical and syntactical levels. The improvements of the EG are statistically significantly higher compared to the control group's performance (exception: Subtest 1, P-ITPA). Consequently, the trial proves the advantages of this strategy therapy compared to the CG, finding that the Lexicon Pirate is an effective approach for treating lexical disorders of school-age students.