After the primary diagnosis of an adenocarcinoma, diagnosis of the origin of a second adenocarcinoma is a problem (primitive or metastatic). Although the clinical evolution or location sometimes makes it possible to determine the origin of the new lesion, in some cases the diagnosis cannot be confirmed. Although a pathological examination is essential for diagnosis, it may still be inconclusive, making management difficult. The use of molecular biology can help solve this problem. We report the case of a patient who presented with an ovarian adenocarcinoma of an undetermined origin one year after an adenocarcinoma of the colon. This clinical case illustrates the contribution of molecular biology in the diagnosis of the origin of an ovarian adenocarcinoma by characterizing allelic losses in 5 chromosome segments using microsatellite markers genotyping in the two lesions. The comparative analysis suggested the primitive origin of the ovarian lesion.