['To study the effect of 16 Gy radiotherapy (RT) vs. 20 Gy RT on Leydig cell function in men treated with radiotherapy against carcinoma in situ (CIS) of the testis.', 'Fifty-one men who were treated between 1985 and 2005 were included. Fourteen men had been treated with 20 Gy and 37 with 16 Gy RT. Measurements of sex hormone-binding globulin and basic and stimulated testosterone, as well as luteinizing hormone levels were performed.', 'The follow-up periods for the patients treated without additional chemotherapy were for the 20 Gy and 16 Gy group mean/median/min-max: 9.0/10.0/1.0-20.3 years and 4.0/3.1/0.4-14.1 years, respectively. During the follow-up period, men treated with 16 Gy RT had stable testosterone levels (-1.1%/year, p = 0.4), whereas men treated with 20 Gy had an annual decrease of 2.4% (p = 0.008). For the latter group, the testosterone decrease was most pronounced in the first 5 years, leveling off during the following 5 years. Additionally, more men treated with 20 Gy needed androgen substitution treatment. Our study showed an increased luteinizing hormone level for the men treated with 16 Gy, although this was not significant (p = 0.5). We anticipated a similar increase in the patients treated with 20 Gy but instead observed a decrease (-3.1%, p = 0.01).'] Is testosterone production better preserved after 16 than 20 Gray irradiation treatment against testicular carcinoma in situ cells?