Objective Hodgkin's lymphoma survivors who were treated with infradiaphragmatic radiotherapy or procarbazine-containing chemotherapy have a fivefold increased risk of developing colorectal cancer (CRC). This study aims to provide insight into the development of therapy-related CRC (t-CRC) by evaluating histopathological and molecular characteristics. Design 54 t-CRCs diagnosed in a Hodgkin's lymphoma survivor cohort were analysed for mismatch repair (MMR) proteins by immunohistochemistry, microsatellite instability (MSI) and KRAS/BRAF mutations. MSI t-CRCs were evaluated for promoter methylation and mutations in MMR genes. Pathogenicity of MMR gene mutations was evaluated by in silico predictions and functional analyses. Frequencies were compared with a general population cohort of CRC (n=1111). Results KRAS and BRAF mutations were present in 41% and 15% t-CRCs, respectively. Compared with CRCs in the general population, t-CRCs had a higher MSI frequency (24% vs 11%, p=0.003) and more frequent loss of MSH2/MSH6 staining (13% vs 1%, p<0.001). Loss of MLH1/PMS2 staining and MLH1 promoter methylation were equally common in t-CRCs and the general population. In MSI CRCs without MLH1 promoter methylation, double somatic MMR gene mutations (or loss of heterozygosity as second hit) were detected in 7/10 (70%) t-CRCs and 8/36 (22%) CRCs in the general population (p=0.008). These MMR gene mutations in t-CRCs were classified as pathogenic. MSI t-CRC cases could not be ascribed to Lynch syndrome. Conclusions We have demonstrated a higher frequency of MSI among t-CRCs, which results from somatic MMR gene mutations. This suggests a novel association of somatic MMR gene mutations with prior anticancer treatment.