We investigated whether the clinical benefits of mindfulness training are mediated by a reduction in maladaptive rumination and an increase in adaptive rumination. Participants in eight-session mindfulness training were assessed before and after treatment, while waiting-list controls were assessed at similar times. For the mindfulness training, a treatment manual was derived from the mindfulness-based cognitive therapy procedure and adapted to account for the consequences of stress, anxiety, and depression rather than just depression. Replicating previous findings, results indicate that mindfulness training, as compared to the waiting-list condition, reduces general psychopathology. It was also found that mindfulness training reduces maladaptive rumination and increases adaptive rumination. Mediational analyses further suggest that clinical benefits of mindfulness required changes in ruminative mode of processing. Results are discussed in terms of Teasdale et al.’s assumption that mindfulness training is effective because it reduces maladaptive rumination.