Adrenocortical carcinoma (ACC) is still one of the most devastating human tumors with a five year survival as low as 20 %. In a previous study, we showed that DNA vaccination followed by vaccinia virus was able to break immune tolerance against murine steroidogenic acute regulatory (mStAR). Prophylactic vaccination in syngenic mice resulted in protective immunity against Sp2-0 tumor cells expressing mStAR. However, approximately a third of the animals developed tumors despite vaccination. This prompted us to investigate whether vaccination failure is responsible for this phenomenon. BALB/cBALB/c mice (in groups of 6 - 9 animals) were vaccinated intramuscularly by injection of cDNA expression vectors encoding mStAR three times at weekly intervals. This was followed by a recombinant vaccinia virus (rVV-mStAR) infection to boost immune response. Ten days after the last vaccination, Sp2-mStAR or parental Sp2-0 cells (as controls) were injected s. c. Tumor development was monitored by daily palpation. Approximately two weeks later, the animals were sacrificed and the spleens removed. After restimulation with the cell lines expressing mStAR, the splenocytes were tested for presence of mStAR self-reactive cytotoxic T-lymphocytes using ELISPOT analysis. With this approach, we were able to show that those animals protected from tumor growth had a specific T-cell response against StAR whereas mice without a specific T-cell response developed Sp2-mStAR tumors. Our data demonstrate that vaccination failure, probably due to the low antigenicity of mStAR, is responsible for tumor growth in our model system.