Summary This study was undertaken to evaluate conventional and some of the main bio‐functional spermatozoa parameters, serum gonadal hormones and didymo‐epididymal ultrasound features in patients with type 1 diabetes mellitus (DM1). DM1 affects an increasing number of men of reproductive age. Diabetes may affect male reproduction by acting on the hypothalamic–pituitary–testicular axis, causing sexual dysfunction or disrupting male accessory gland function. However, data on spermatozoa parameters and other aspects of the reproductive function in these patients are scanty. Thirty‐two patients with DM1 27.0 (25.0–30.0 years) and 20 age‐matched fertile healthy men 28.0 (27.25–30.75 years) were enrolled. Patients with diabetic neuropathy, other endocrine disorders or conditions known to alter spermatozoa parameters were excluded. Each subject underwent semen analysis, blood withdrawal for fasting and post‐prandial glycaemia, hormonal analysis and didymo‐epididymal ultrasound evaluation before and after ejaculation. Patients with DM1 had a lower percentage of spermatozoa with progressive motility 10.0 (7.0–12.75) vs. 45.0 (42.0–47.75) %; p < 0.01 and a higher percentage of spermatozoa with abnormal mitochondrial function than controls 47.0 (43.0–55.0) vs. 2.0 (1.0–5.0) %; p < 0.01. Patients also had greater post‐ejaculatory diameters of cephalic 11.5 (10.2–13.6) vs. 6.0 (4.0–7.0) mm; p < 0.01 and caudal epididymis 5.5 (4.00–7.55) vs. 3.0 (2.0–4.0) mm; p < 0.01 compared to controls, suggesting a lack of the physiological post‐ejaculation epididymal shrinkage. Correlation analysis suggested that progressive motility was associated with fasting glucose (r = −0.68; p < 0.01). The other parameters did not show any significant difference. Patients with DM1 had a lower percentage of spermatozoa with progressive motility, impaired mitochondrial function and epididymal post‐ejaculatory dysfunction. These findings may explain why patients with DM1 experience fertility disturbance. Larger multi‐centric studies are necessary to confirm these results.