We made a retrospective study to determine useful parameters for predicting subfertility in patients with subclinical varicocele (SV). One hundred and fifty men with SV and 17 age-matched men without SV were divided into three groups: Group 1, subfertile patients (n = 15); Group 2, fertile patients (n = 135); and Group 3, control patients (n = 17). Their age, body mass index (BMI), semen analysis, scrotal temperature, testicular volume, resistive index (RI), pulsatility index (PI) and peak retrograde flow (PRF) were compared. Subfertile patients (Group 1) with SV had significantly lower testicular volume and higher scrotal temperature, RI, PI and PRF than fertile men with SV (Group 2) and the control group (Group 3). Elderly men (>50 years, n = 30) with SV had a significantly higher incidence of bilateral SV than young men with SV (10/30; 33.3% vs. 12/120; 10%). There was no difference in age and BMI among the three groups. Patients with SV and RI >0.55 ml/s, PI >0.99 ml/s, total testicular volume <27 cc, scrotal temperature >34.9°C and PRF >29 cm/s have higher incidence of subfertility. Patients with SV may suffer from subfertility regardless of age. Close follow-up with colour Doppler ultrasound may be beneficial.