Multiple synchronous primary malignancies have been reported in colon carcinoma and studied for lynch syndrome or constitutional mismatch repair deficiency syndrome, however we present first case of co-occurance of colorectal adenocarcinoma and wilms tumor with mismatch repair deficiency. A 17 year old girl presented with pain in abdomen. A computerized tomography of whole abdomen revealed a large left sided renal mass along with mass lesion seen in the rectum Histopathology from both the mass revealed presence of colorectal adenocarcinoma and wilms tumor in renal mass. Immunohistochemistry for microsatellite instability was performed which showed loss of expression for MSH2/MSH6 and absence of BRAF mutation with presence of KRAS mutation. Diagnosis of wilms tumor with colorectal adenocarcinoma was made and possibility of constitutional mismatch repair deficiency was considered. A multidisciplinary approach is required for diagnosing synchronous malignancies and apart from clinical, radiological and histopathological studies, microsatellite instability testing in patients with colorectal carcinoma should also be considered to rule out Lynch syndrome or Constitutional Mismatch Repair Deficiency Syndrome. This report describes for the first time the coexistence of a colon carcinoma with Wilms tumor.