Background: Testicular cancer (TC) treatment leaves many patients with low levels of testosterone. While most TC patients with low testosterone (< - 2 SD) and hypogonadal symptoms will initiate testosterone replacement therapy (TRT), the role of TRT in patients with mild Leydig cell insufficiency, defined as elevated luteinizing hormone in combination with borderline low testosterone, is unknown. To clarify if TRT improves symptoms of depression and anxiety, sexual function, fatigue, and quality of life in TC survivors with mild Leydig cell insufficiency. Materials and Methods: In total, 69 men aged between 18 and 65 years with mild Leydig cell insufficiency after TC treatment were randomized 1:1 to 12 months daily transdermal testosterone (maximum dose 40 mg/daily) vs. placebo. Patient reported anxiety, depression, sexual function, fatigue, and overall quality of life were assessed at baseline, after 6- and 12 months treatment, and 3 months post-treatment using validated questionnaires. Results: After 12 months of treatment, median luteinizing hormone and median free testosterone were normalized in the testosterone group. Compared to placebo, TRT was not associated with statistically significant improvement of symptoms of anxiety and depression, sexual function, fatigue, and overall quality of life. Testosterone replacement therapy did not improve anxiety, depression, sexual function, fatigue, or overall quality of life in patients with mild Leydig cell insufficiency compared to placebo. Conclusion: Routine TRT in TC survivors with mild Leydig cell insufficiency to improve sexual function and quality of life cannot be generally recommended. The findings should preferably be validated in a larger cohort.