Malignant non-Hodgkin's lymphoma (MNHL) is a frequent tumour but a primary intra-muscular location is exceptional. Standard treatment combines wide surgical removal with chemotherapy and radiotherapy. The functional consequences of this large excision are sometimes very unsatisfactory. We report the case of an intra-muscular MNHL of the posterior and internal part of the thigh. The diagnostic was established thanks to selective biopsies. The treatment was purely medical and conservative with chemotherapy after a multidisciplinary oncological discussion. Three years after the treatment, the general state of health is excellent with no signs of local or remote recurrence.