Up to 64% of cancer patients complement their therapy with Viscum album (VA) treatment at some stage of their disease. This very common complementary and alternative medicine treatment is frequently used to mitigate adverse drug reactions of chemo and radio therapy, as well as to increase quality of life. However, it is currently unclear if this co-medication has detrimental effects on the patients. Building on data of the Network Oncology (NO) cancer register, the following investigation compares colorectal cancer (CRC) patients, treated with or without VA.Since 2001, the NO systematically collects and standardises all data on file of all cancer patients treated in Gemeinschaftskrankenhaus Havelhöhe (GKH). All cases of CRC from 1996 until 2001 were included retrospectively. All patients were contacted in 2001/2, 2004 and 2007 by mail to obtain information on disease progression and therapeutic interventions, subsequent to their visit of GKH. Where necessary, a reminder was sent 8 weeks later. Whenever informed consent was given, their GP or local oncologist was also contacted. Mortality rates and time of death were calculated using data from deaths’ registries.Thus far 925 patients have been included (57.5% female). Average age of first diagnosis was 69 (71 female, 67 male). Of the total number, 697 were followed up or confirmed dead by 2007. Return rates by mail were approximately 60% by patients and 40% by GPs/oncologists. The majority of patients was treated according to standard guidelines at the time of consultation. About 43% of males and 54% of females received also VA between 4 weeks and several years. Early indications suggest that the median survival time of VA-treated patients was 29 month, respectively 22 month in the controls. However, average age of first diagnosis was lower (67 and 72, respectively). When adjusted for age, sex and disease state a trend for survival improvement in the viscum group is seen.Using NO as a basis, it is possible to follow up all patients with a specific diagnosis or treatment modality. The present cohort study demonstrates that NO is well suited to carry out health service research. If VA has an effect on survival time remains a question for further follow up.