The authors explored the group as a source of change in mindfulness-based stress reduction (MBSR). Participants consisted of 606 adults in 59 groups who completed an 8-week MBSR program. The authors examined change in the General Symptom Index (GSI) of the Symptom Checklist-90-Revised and the Medical Symptom Checklist (MSC) from pre- to postintervention. Multilevel models were used to examine the extent to which groups differed in the amount of change reported by the participants. After controlling for pretreatment severity, group accounted for 7% of the variability in the GSI and 0% in the MSC. The authors discuss the implications of these findings for the practice of MBSR as well as for research investigating the effects of MBSR and other programs or psychotherapies.