We describe a 22-year-old patient with Klinefelter's syndrome associated with unilateral renal aplasia. Unilateral absence of kidney was detected in our ongoing screening for renal abnormalities by renal ultrasound in male hypogonadism. The present abnormality may be a co-existing entity or previously unrecognized abnormality associated with Klinefelter's syndrome since there is not reported systematic screening for urinary tract abnormalities in patients with Klinefelter's syndrome in the literature.