The present meta analysis summarized the results of 37 published, controlled studies that investigated the effectiveness of psychosocial interventions on quality of life (QoL) in adult cancer patients. The overall effect size of psychosocial interventions and the effect of potential moderating variables such as type and duration of intervention, sociodemographic and clinical parameters, characteristics of QoL measurement, and methodological quality of the selected studies were calculated using a meta analysis model suggested by Hunter and Schmidt. The overall effect size was 0.31 (correlation equivalent delta) which corresponds to a standardized mean difference of d=0.65 (N=3120 cancer patients). The most important moderating variable was duration of psychosocial intervention with durations of more than 12 weeks being significantly more effective than interventions of shorter duration. The meta analytical findings support the usefulness of psychosocial interventions for improving QoL in adult cancer patients.