We prospectively performed sural and radial sensory nerve conduction studies in 92 healthy subjects, aged between 21 and 88 years, both to determine the lower limits of normal (LLN) and to assess the effects of age and body mass index (BMI) on the sural and radial sensory nerve action potential (SNAP) amplitudes and on the sural/radial amplitude ratio (SRAR). Using the nonparametric bootstrap method to calculate 95% confidence intervals, we found that the 5% LLN values for sural and radial SNAPs were 14 microV and 25.5 microV in subjects aged < or =39 years, 7 microV and 17.4 microV in subjects aged 40-59 years, and 3 microV and 12 microV in subjects aged > or =60 years. The 5% LLN for SRAR for all patients was 0.21. Sural and radial SNAP amplitudes but not SRAR were strongly and inversely correlated with age and BMI. These age-adjusted normal values and revised SRAR will aid in the electrodiagnosis of polyneuropathy.