The medical treatment of retentio testis remains controversial because of ineffectiveness and/or adverse events. Follicle-stimulating hormone (FSH) seems to influence the spontaneous descent of the testis; furthermore, it induces luteinizing hormone (LH) receptors. Therefore, we performed a double-blind placebo-controlled study to investigate the effect of FSH with human chorionic gonadotrophin (hCG) versus hCG alone in retentio testis patients. Twenty-two boys with retentio testis were investigated, excluding retractile testis. Group A (N = 14: four with bilateral and 10 with unilateral retentio testis; mean age 3.15 years) was treated with 150 IU of FSH twice a week for 2 weeks followed by 150 IU of FSH and 250 IU of hCG (half the recommended World Health Organization dose) twice a week for another 4 weeks. Group B (N = 8: two with bilateral and six with unilateral retentito testis; mean age 3.3 years) was treated with 250 IU of hCG twice a week for 6 weeks. Testicular position, volume and consistency as well as the appearance of the scrotum and the penile length were determined at the start of the treatment as well as at weeks 2, 4, 6 and 12 by two independent investigators. Blood investigation consisted of measurements of LH, FSH, testosterone and sex hormone-binding globulin. Successful descent was considered when the testis reached a mid- or low scrotal position. In group A, 6/18 testes descended successfully. In group B, 6/10 testes descended. Of the unsuccessfully treated patients, six of group A and three of group B underwent surgery.(ABSTRACT TRUNCATED AT 250 WORDS)