The prevalence of sleep disturbance is high among cancer patients, and the sleep problems tend to last for years after the end of treatment. As part of a large randomized controlled clinical trial (the MICA trial, NCT00990977) of the effect of mindfulness-based stress reduction (MBSR) on psychological and somatic symptoms among breast cancer patients, the aim of the current study was to evaluate the effect of MBSR on the secondary outcome, 'sleep quality'. Material and methods. A total of 336 women operated on for breast cancer stage I-III 3-18 months previously were randomized to MBSR (n = 168) or treatment as usual (n = 168); both groups received standard clinical care. The intervention consisted of an eight-week MBSR program (psycho-education, meditation and gentle yoga). Sleep quality was assessed on the Medical Outcome Study sleep scale at baseline, after the intervention and at six-and 12-months' follow-up. Results. The mean sleep problem scores were significantly lower in the MBSR group than in controls immediately after the intervention. Quantile regression analyses showed that the effect was statistically significant only for the participants represented by the lower percentile of change between baseline and post-intervention, i.e. those who had more sleep problems; the MBSR group had a significantly smaller increase in sleep problems than the control group. After the 12-month follow-up, there was no significant between-group effect of MBSR on sleep quality in intention-to-treat analyses. Conclusion. MBSR had a statistically significant effect on sleep quality just after the intervention but no long-term effect among breast cancer patients. Future trials in which participation is restricted to patients with significant sleep problems are recommended for evaluating the effect of MBSR on sleep quality.