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Unilateral orchiectomy (UO) in adult bonnet monkeys and boars elicits a compensatory increase in size and sperm production of the remaining testis. The objective of this study was to investigate whether a similar effect is evident also in humans. We prospectively studied 50 patients from October 2003 to December 2005 who underwent UO for seminomatous tumour, with sperm concentration >20 × 106/mL or total sperm count >40 × 106 at diagnosis and without elevation of serum tumour markers. Patients were followed-up with surveillance and they were studied at the time of diagnosis of testicular cancer (T−1), 1 month after unilateral orchiectomy (T0) and yearly for 3 years (T1, T2, T3) with semen analysis, measurement of plasma levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), inhibin B, total testosterone, and oestradiol and ultrasonographic scanning of the remaining testis. A decline in circulating inhibin B and an increase in FSH levels were evident 1 month after UO. The elevation of FSH was maintained up to 3 years and was associated with a significant increase in testicular volume of 19 and 30%, 2 and 3 years after UO respectively. Although patients had normozoospermia at the time of diagnosis of testicular cancer, they showed a statistically significant increase in total sperm count at T2 and T3 with respect to T−1 and T0. In conclusion, we showed that in humans, the testes are not normally operating at their maximal potential in terms of spermatogenesis. Therefore, in physiological situations, FSH secretion is insufficient to stimulate spermatogenesis to its ceiling. A sustained endogenous increase in FSH secretion might drive human testes towards their maximal function.