To examine the significance of underweight and physical function as well as their interaction on mortality in the aged.Prospective cohort.The Elderly Nutrition and Health Survey in Taiwan during 1999-2000.Total of 1435 representative free-living elders (739 men and 696 women).Body composition was assessed by various anthropometrics. Physical function score (PF, ranged 0-100) was derived from the SF-36(®). Death by December 31, 2006 was the outcome measure.After 7.9 (median: 7.0) years follow-up, 381 (223 men, 158 women) of 1435 eligible participants had died. Those with the lowest PF (<45) had 3.43 (hazards ratio (HR), 95% confidence interval (CI) = 2.20-5.36) times the all-cause mortality risk of the highest PF (≥58). Interactions for PF and BMI (P =0.02) and for PF and wrist circumference (P =0.09) on death were found after controlling for potential confounders. Jointly, compared to normal-BMI-highest-PF, the greatest HR for death occurred where BMI <18.5 kg/m2 was combined with the lowest-PF after covariate adjustments (HR = 8.67, 95% CI = 3.77-20.0). Similarly, the lowest arm muscle circumference (MAMC)-PF had a HR of 5.22 compared to mid-MAMC-highest-PF. However, percent and absolute body fat, estimated by bioelectrical impedance, was comparable to non-sarcopenic individuals.Thin elderly Taiwanese with sarcopenia, and less skeleton, are at the most risk of death, especially if physical function is limited.