A small hard lump growing behind the left testicle may be epididymal cyst, inflammatory mass, testicular nodule, testicular tumor and so on. 1. Epididymal cyst: due to the partial blockage of the vas deferens in the left epididymis and other reasons, the symptoms of cyst may appear, manifested as a small hard lump growing behind the left testicle. 2. Inflammatory mass: due to pathogen infection of the epididymis testis, an inflammatory mass may appear, which may manifest as a small hard lump. 3. Testicular nodules: if the testicular trauma leads to the formation of hard calcified nodules after healing. 4. Testicular tumors: such as testicular adenomatoid tumors, commonly found on the epididymis, which may protrude from the testicular surface. It is recommended that the person who has a small hard lump at the back of the left testicle should go to the regular hospital in time to clarify the cause of the disease and carry out standardized treatment under the guidance of the doctor.