Agrammatism is—among others, characterized by a deficit in producing grammatical structures. Of specific difficulty is the utilization of complex, non-canonical sentence structures (e.g. object-questions, passives, object-clefts). Several studies have documented positive effects when applying a specific treatment protocol in terms of increasingly correct production of target complex sentence structures with some variance in generalization patterns noted across individuals. The objective of this intervention study was to evaluate an intervention program focussing on the production of non-canonical sentences. Hypotheses about the occurrence of treatment effects were formulated on the basis of syntactic complexity, referring to the amount of syntactic phrase structures necessary to generate specific German sentence structures. A multiple single case study with seven agrammatic participants was applied, each participant receiving training in the production of object-relative-clauses and who-questions. The investigation was designed to unambiguously evaluate for each individual, structure specific and generalized learning effects with respect to the production of object-relative-clauses, who-questions and passive sentences. Results showed significant improvements for all sentences types. This outcome is considered within methodological issues of treatment studies. Theoretical and clinical implications are discussed.