BACKGROUND: Mistletoe is often used as a complementary approach in oncology. Despite experimental anti-tumour effects and several reviews there remains controversy about its clinical role. PATIENTS AND METHODS: Potentially relevant trials were identified to perform a systematic review (databases: e.g. EMBASE, MEDLINE; hand search: e.g. bibliographies; search terms: e.g. mistletoe). To be included, randomised or comparative clinical trials at least had to examine mistletoe preparations standardized according to manufacturing process and to describe interventions explicitly. Additionally, cohort studies were included for reasons of external validity. Results were summarised in tables. RESULTS: 18 clinical trials (>6,800 participants) were included. Their internal quality was mostly low. Due to heterogeneity between trials a meta-analysis was impossible. Regarding efficacy, findings were inconsistent regarding life expectancy, relation to tumour entity, dosing and treatment duration. Yet, studies indicate that quality of life (QoL) is improved. As these findings do not seem to be limited to one of the different parenteral mistletoe preparations reviewed the treatment may be summarised under the umbrella term 'mistletoe therapy'. Regarding safety, 1 serious adverse event (AE) related to mistletoe was described; non-serious AEs were local reactions at injection site. Allergic reactions were rare. CONCLUSION: Supportive 'mistletoe therapy' seems safe and beneficial for QoL in adult patients with solid tumours. But there is an urgent need to confirm its efficacy in patient-centred care in a complex oncological setting. This has to be evaluated systematically in prospective observational trials with validated, multidimensional patient-rated QoL questionnaires and comparisons of different preparations and dosages. Copyright 2009 S. Karger AG, Basel.