Mindfulness-based interventions have been incorporated into a variety of psychotherapies. Attentional disruptions are common in many mental disorders, and it seems generally accepted that practicing mindfulness enhances attention. We tested the hypothesis that mindfulness training would enhance four components of attention: sustained vigilance, concentration, inhibition of distraction, and executive control. A randomized three-group design included: (1) a mindfulness meditation group, (2) a progressive muscle relaxation group to control for effects of physical relaxation on attention, (3) a wait-listed group to control for practice effects of repeated measures. Fifty-three community adults were randomly assigned to one of these groups. Forty-five participants completed the 4-week program. After training and 4 weeks of twice-daily practice, the mindfulness group demonstrated significantly greater discriminability on a signal detection task than did the other groups. Significant improvements in sustained attention were found following mindfulness meditation, which did not appear to be mediated by relaxation or practice effects. Performances on measures of concentration and inhibition of distraction did not support the hypothesis. These results partially support current considerations of mindfulness meditation to enhance attention.