This article describes the features of the nutrient database and analysis system developed for the Modification of Diet in Renal Disease (MDRD) Study and outlines the development process itself, an example of tailoring a database and analysis system for a complex, long-term clinical trial. The MDRD Study was a 15-center trial for men and women aged 18 to 70 years with chronic renal disease who were not on dialysis and who had not had a kidney transplant. The effects of three levels of protein and phosphorus intake and two levels of blood pressure management were assessed on the rate of decline in kidney function. This paper describes the development, structure, contents, and application of the nutrient database and analysis system that was used to enter, analyze, and report the MDRD Study dietary data.