The aim of this study was to investigate predictors of the improvement of semen characteristics after low ligation for patients with varicoceles. The records of 97 oligozoospermic patients who underwent microsurgical left or bilateral inguinal varicocele repair were retrospectively evaluated. We assessed factors that could be predictors of the improvement of semen characteristics using logistic regression analysis. We evaluated age, testicular volume, varicocele grade, serum follicle-stimulating hormone (FSH), luteinising hormone, testosterone, sperm concentration and motility before low ligation. Among the 97 patients, sperm concentration improved from 6.4 +/- 5.8 to 24.2 +/- 35.1 million ml(-1) and sperm motility increased from 32.8 +/- 24.9% to 41.0 +/- 14.6% in 55 patients (57%). In logistic regression analysis, pre-operative serum FSH and testosterone concentration were predictors of sperm concentration improvement. Varicocele repair improved sperm concentration and motility. Low serum FSH and high testosterone are significant factors predicting the improvement of semen characteristics before low ligation.