Purpose: Group conversation treatment has the potential to improve communication and reduce social isolation for people with aphasia. This project examined how 2 conflicting hypotheses--treatment dosage and group dynamics--affect treatment outcomes. Method: Forty-eight participants with chronic aphasia were randomly assigned to either a dyad, a large group, or a delayed control group. Conversation group treatment was provided for an hour, twice per week, for 10 weeks. Individual goals were developed by each participant and addressed in the context of thematically oriented conversation treatment. Standardized testing across language domains was completed pretreatment (Time 1), posttreatment (Time 2), at a 6-week maintenance point (Time 3), and at 11-month follow-up for the experimental groups. Results: Treatment groups showed greater changes on standardized measures than the control group posttreatment. Dyads showed the most changes on measures of language impairment, whereas changes on the self-reported functional communication measure (Aphasia Communication Outcome Measure) and connected speech task only showed significant changes in the large group. Conclusions: This randomized controlled trial on conversation treatment indicated that both treatment groups--but not the delayed control group--showed significant changes on standardized tests. Hence, conversation treatment is associated with changes in measures of language impairment and quality of life. Dyads showed the most changes on measures of language impairment, whereas changes on the functional communication measure (Aphasia Communication Outcome Measure) and discourse production only showed significant changes in the large group. Thus, group size may be associated with effects on different types of outcome measures.