Aim: Evaluations of sacs and cremaster muscles associated with undescended testis (UT) have revealed a decrease in sympathetic, but an increase in parasympathetic tonuses. Since the low frequency/high frequency power ratio obtained through spectral analysis of heart rate variability (HRV) determines the sympatho-parasympathetic balance, the sympatho-parasympathetic balance as defined by HRV in healthy boys and in boys with UT were compared.