Objective The aim of this study was to examine whether estimated net acid excretion (NAE) from two dietary assessment instruments can predict urine pH in healthy volunteers. Design The Observing Protein and Energy Nutrition (OPEN) study, which was designed to assess dietary measurement error, included men and women aged 40 to 69 years. Participants were asked to complete two food frequency questionnaires (FFQs) and two 24-hour recalls at approximately three-month intervals, and to return two 24-hour urine samples during the same period. NAE was estimated using a previously described equation. The estimates obtained from the different dietary assessment measures were correlated with actual urine pH. Subjects/Setting Four hundred eighty-four subjects were enrolled in the OPEN study. All participants were included in this study on urine pH, given that they had all completed at least one dietary assessment and one urine sample. The study took place in Montgomery County, MD, between September 1999 and March 2000. Results Correlation coefficients between NAE estimates using the FFQ data and the average of two urine pH measures were −0.34 and −0.39 for women and men, respectively. Removing within-person variance in the urine pH levels resulted in a deattenuated overall correlation of −0.50 for the FFQ. Correlations between the 24-hour recall estimates of NAE were −0.41 and −0.32 for women and men, respectively. For the 24-hour recalls, the deattenuated overall correlation was −0.50. Application/Conclusions Our findings suggest that FFQs provide adequate dietary data to estimate renal NAE and could be used as a surrogate for urinary pH levels in large epidemiologic studies. J Am Diet Assoc. 2003;103:1001-1007.