Objective. The aim of this study was to investigate the effects of acute exercise on the diameter of the spermatic vein, and on the duration of reflux in patients with varicocele. Material and methods. The study included 38 patients with complaints of infertility and scrotal pain between 2009 and 2010. The diagnoses were made by physical examination and colour Doppler ultrasound, with both performed before and after exercise tests. Results. The mean age of the participants was 25.7 +/- 4.9 years. During the first examination, the grades of the varicoceles detected were as follows: grade I, n = 7; II, n = 10; and III, n = 21. The diameters of veins in patients with grades I, II and III varicocele were 2.1 mm, 2.9 mm and 4.2 mm, respectively, before exercise, whereas they were 2.6 mm, 3.2 mm and 4.3 nun, respectively, after exercise. In patients with grade I varicoceles, compared with pre-exercise values, the diameter of the left spermatic vein and duration of reflux measured during Valsalva manoeuvres were increased significantly after exercise (p = 0.042 and p = 0.034, respectively); similar results were obtained for the patients with grade II varicoceles (p = 0.007 and p = 0.008, respectively). However, the minimal relative increase in cases with grade III varicoceles was not statistically significant (p > 0.05). Conclusions. This study demonstrates that acute exercise increases the spermatic vein diameter and reflux time in patients with varicoceles. These outcomes demonstrate that acute exercise may be an aggravating factor for varicocele, as seen in chronic exercise.