We studied the semen quality and plasma testosterone levels (T) in 32 adolescent patients with insulin-dependent diabetes mellitus and in an aged-matched control group. Semen volume, motility and morphology were significantly lower in diabetics whereas seminal fructose and glucose were significantly higher. Even though the sperm count was lower in these adolescent diabetics, the difference was not significant when compared to the control group. No difference was observed in plasma testosterone levels. Patients were divided into two groups according to the presence of retinopathy and neuropathy, and degree of metabolic control. Spermiogram parameters, seminal fructose and glucose were lower in diabetics with neuropathy. No difference was observed in spermiogram parameters between diabetic patients with or without retinopathy, but seminal fructose, and glucose were lower in the former. All spermiogram parameters, as well as seminal fructose were lower in diabetics with poor metabolic control but seminal glucose was higher. No correlation was detected between clinical parameters (age at onset and duration of diabetes mellitus and time since first ejaculation), semen parameters, plasma T, glycemia and glycosuria. In conclusion, a deterioration of the quality of human semen occurs in adolescent diabetic patients. Neuropathy and poor metabolic control seem to be important factors of this deterioration. The presence of retinopathy does not correlate with T and semen quality.