BACKGROUND: Recent research suggests that the complexity of treatment stimuli influences the effectiveness of treatment. However, no studies have examined the role of complexity on sound production treatment in adult individuals with sound production impairments. AIMS: This study examines effects of syllable complexity on treatment outcome in two patients with acquired sound production problems. Complexity is defined in terms of syllable structure: clusters are more complex than singletons. Using a single-subject multiplebaseline design, we address the question: Is treatment of complex syllables more effective than treatment of simple syllables? METHODS AND PROCEDURES: Two patients with aphasia and apraxia of speech were trained to produce complex or simple syllables (using modelling). Improvement was measured by percent correct on a word and nonword repetition test. OUTCOMES AND RESULTS: We found that both treatment on simple syllables and treatment on complex syllables led to improved production of simple syllables, while treatment of complex syllables also led to improvement on some complex syllables for one of the two patients. CONCLUSIONS: These results suggests that training complex items is more effective than training simple items, at least for some patients. Possible reasons for lack of stronger effects are discussed, as well as directions for future research.