The Modification of Diet in Renal Disease Study is randomized, multicenter, clinical trial designed to determine the effects of three levels of dietary control of protein and phosphorus and two levels of blood pressure control on the rate of decline of kidney function among persons with chronic renal disease. Study participants were assigned to one of two studies, Study A or Study B, depending on their GFR just before randomization. Within each study, participants were randomly allocated to one of two levels of blood pressure control and to one of two dietary interventions according to separate 2 x 2 factorial designs. A total of 840 men and women aged 18 to 70 were randomized. This report summarizes the demographic, biochemical, and clinical characteristics of the randomized participants at the time of entry into the trail, overviews the protocol and purposes of the baseline period before randomization, and evaluates the balance among the treatment intervention groups within Studies A and B at the time of randomization. Major indicators of renal function were found to be well balanced among the treatment groups. Selected baseline characteristics of participants in the Modification of Diet in Renal Disease Study are compared with those of other renal clinical trials and with those of new cases of treated ESRD reported in the United States Renal Data System.