We examined the impact of low birth weight and low current body weight on proteinuria in a cohort of children participating in the pilot study of a nationwide screening program. Two thousand eighty-three children underwent screening examinations, including birth history, anthropometric measures, blood pressure measurements, and urinalysis. On this study, children with proteinuria were found to have significantly lower mean body weights compared with children without proteinuria (38.7 +/- 7.6 versus 42.8 +/- 11.0 kg; P < 0.001). Progressively decreasing body weights were associated with increasing degrees of proteinuria (42.8 +/- 11.0, 38.9 +/- 7.6, and 37.2 +/- 8.5 kg for 0, 30, and 100 mg/dL of protein, respectively; P = 0.05). When examined by multiple logistic regression, low body weight was associated with a 1.8-fold greater risk (95% confidence interval, 1.27 to 2.64; P = 0.0019) for proteinuria after adjusting for potential confounders. There were trends for lower birth weights in children with proteinuria (3,047.6 +/- 445.2 versus 3,175.0 +/- 608.6 g for proteinuric and nonproteinuric groups, respectively; P = 0.275) and a greater prevalence of children with birth weights less than the 25th percentile (31.3% versus 25.0%; P = 0.786). The relationship between low current body weight and proteinuria was not explained by differences in blood pressure. In conclusion, low current body weight had a stronger relationship with proteinuria than low birth weight in this pediatric population. We hypothesize that the effect of low birth weight on renal disease may be significantly enhanced by environmental factors that result in a low current body weight.