The prevalence of varicocele was estimated among pupils of forms 4 and 8 in the city of Hamburg and the severity of the associated venous reflux was analysed. In the school year of 1998/99, a genital examination was performed on 2756 children (median age 10.2 years) and 2008 adolescents (14.6 years). The varicocele degree was determined according to the WHO. Venous reflux was proven by CW-Doppler sonography, distinguishing between Valsalva-induced reflux (VR) and continuous reflux (CR). As a result, varicoceles were detected in 18.0% of the children (1.2% bilaterally) and 42.7% of the adolescents (7.2%). High-graded forms (palpable/visible) occurred with increasing rate (from 7% to 22.9%) on the left side. Subtle forms (subclinical/during Valsalva) counted for >90% of all right-sided findings, whilst an age-related shift towards higher degrees was noted left-sided. VR occurred bilaterally, CR was almost only established left-sided. VR was mainly associated with subtle varicoceles, CR was predominantly found in the high-graded forms. These results suggest that even in children varicoceles are not a rare phenomenon. However, adolescence is the main period of manifestation. A major venous malfunction is already evident in maturing boys, which seems to be associated with the formation of high-graded varicoceles.