Objective: The use of mindfulness-based therapy (MBT) in oncology settings has become increasingly popular, and research in the field has rapidly expanded. The objective was by means of a systematic review and meta-analysis to evaluate the current evidence for the effect of MBT on symptoms of anxiety and depression in adult cancer patients and survivors. Method: Electronic databases were searched, and researchers were contacted for further relevant studies. Twenty-two independent studies with a total of 1,403 participants were included. Studies were coded for quality (range: 0-4), and overall effect size analyses were performed separately for nonrandomized studies (K = 13, n = 448) and randomized controlled trials (RCTs; K = 9, n = 955). Effect sizes were combined using the random-effects model. Results: In the aggregated sample of nonrandomized studies (average quality score: 0.5), MBT was associated with significantly reduced symptoms of anxiety and depression from pre- to posttreatment corresponding to moderate effect sizes (Hedges\'s g) of 0.60 and 0.42, respectively. The pooled controlled effect sizes (Hedges\'s g) of RCTs (average quality score: 2.9) were 0.37 for anxiety symptoms (p less than 0.001) and 0.44 for symptoms of depression (p less than 0.001). These effect sizes appeared robust. Furthermore, in RCTs, MBT significantly improved mindfulness skills (Hedges\'s g = 0.39). Conclusion: While the overall quality of existing clinical trials varies considerably, there appears to be some positive evidence from relatively high-quality RCTs to support the use of MBT for cancer patients and survivors with symptoms of anxiety and depression. (Contains 2 tables, 1 figure and 3 footnotes.)