Objective: This study evaluated the effectiveness of mindfulness-based cognitive therapy (MBCT) for individuals with a diagnosis of cancer. Method: Participants (N = 115) diagnosed with cancer, across site and stage, were randomly allocated to either the treatment or the wait-list condition. Treatment was conducted at 1 site, by a single therapist, and involved participation in 8 weekly 2-hr sessions that focused on mindfulness. Participants meditated for up to 1 hr daily and attended an additional full-day session during the course. Participants were assessed before treatment and 10 weeks later; this second assessment occurred immediately after completion of the program for the treatment condition. The treatment condition was also assessed at 3 months postintervention. All postinitial assessments were completed by assessors who were blind to treatment allocation. Results: There were large and significant improvements in mindfulness (effect size [ES] = 0.55), depression (ES = 0.83), anxiety (ES = 0.59), and distress (ES = 0.53) as well as a trend for quality of life (ES = 0.30) for MBCT participants compared to those who had not received the training. The wait-list group was assessed before and after receiving the intervention and demonstrated similar change. Conclusions: These improvements represent clinically meaningful change and provide evidence for the provision of MBCT within oncology settings.