Background: High level microsatellite instability (MSI-H) occurs in 15% of colorectal cancers as a consequence of DNA mismatch repair deficiency. MSI-H colorectal cancers are characterized by a dense immune cell infiltration, which is compatible with a pronounced anti-tumoral immune response. MSI-H cancers are characteristic of the inherited HNPCC (hereditary non-polyposis colorectal cancer) or Lynch syndrome, but can also develop sporadically. In MSI-H cancers, DNA mismatch repair deficiency causes insertion or deletion mutations at coding microsatellites, which may lead to shifts of the translational reading frame and to the generation of MMR deficiency-related frameshift peptides (FSPs). Expression of FSP antigens is restricted to MMR-deficient cells; therefore, FSP antigens are promising targets of vaccination approaches. Methods: We have recently initiated a clinical phase I/IIa vaccination trial that evaluates vaccination with a combination of three FSP antigens (derived from frameshift variants of the coding microsatellite-containing genes AIM2, HT001, TAF1B) in the clinical setting. Inclusion criteria allow vaccination of patients with history of metastasized colorectal cancer (UICC stage III or IV) after the end of standard chemotherapy. Phase I of the trial evaluates safety and toxicity as the primary endpoint (6 patients); phase IIa addresses the induction of cellular and humoral immune responses (16 patients). Results: Data from phase I demonstrate that no FSP antigen-associated severe adverse events have been observed after FSP vaccination. Significant induction of FSP-specific T cell immune responses was observed in 3 out of 4 evaluated patients. Antibody responses against FSP antigens were significantly induced in 9 out of 10 evaluated patients. Conclusions: If vaccination with FSPs turns out to be well tolerated and leads to the induction of effector T cell immune responses, FSP vaccination may represent a promising novel approach for treatment of MSI-H colorectal cancer patients and for tumor prevention in Lynch syndrome mutation carriers. Clinical trial information: NCT01461148.