The survival of many patients with advanced solid tumours has remained poor over the past 40 years, despite improvements in existing therapies. Progress in the field of molecular biology has made gene therapy a feasible treatment modality. Viruses are the most promising vectors, with an adenovirus the first to receive a government licence in 2003. However, the efficacy of adenoviral monotherapy has been disappointing in clinical trials. Vaccinia virus is an attractive alternative, with many superior characteristics. There are several strains of vaccinia virus, some of which have established safety profiles following their use as smallpox vaccines in millions of humans. The dominant strain in cancer research, Western Reserve, has been reported as the most potent oncolytic virus for cancer treatment, but is a non-vaccine strain. Other strains, such as the European vaccine Lister strain, are largely untested. This study evaluated the anti-tumour potency and biodistribution of different vaccinia virus strains using in vitro and in vivo models of cancer. Lister strain vaccinia virus demonstrated superior anti-tumour potency and cancer-selective replication in vitro and in vivo, compared to Western Reserve; especially in human cancer cell lines. Studies examining the safety of the virus following local and systemic delivery to immunocompetent mice bearing tumours confirmed favourable viral biodistribution of the Lister strain; with higher viral titres in the tumours. Examination of the serum from a mouse cancer model found lower levels of pro-inflammatory cytokines following Lister strain treatment. Strategies to further improve the Lister strain virus were considered, and a Lister strain virus expressing the anti-inflammatory cytokine IL10 was shown to increase anti-tumour potency and viral titres within treated tumours. This study suggests that the Lister strain is a more promising vaccinia virus strain than the Western Reserve strain for oncolytic viral gene therapy. IL10 expressed as a transgene enhances intratumoural viral persistence and anti-tumour potency.