Many clinical studies of the effects of low-protein and low-phosphorus diets on the course of chronic renal disease have used the rate of decline in renal function to assess the rate of progression. In this report, data from the feasibility phase of the Modification of Diet in Renal Disease Study were used to analyze methods used in other studies. The focus is particularly on the effects of duration of follow-up and of regression to the mean. The findings are summarized as follows. (1) During the mean follow-up period of 14.1 months, rates of decline in glomerular filtration rate, creatinine clearance, and the reciprocal of the serum creatinine concentration were highly variable among individuals, and mean rates of decline were slow. (2) Precision of estimates of individual rates of decline in renal function were relatively low and improved with increasing duration of follow-up. (3) Correlations between rates of decline in creatinine clearance and the reciprocal of the serum creatinine concentration with glomerular filtration rate in individuals were significant but weak and became stronger with increasing duration of follow-up. (4) After entry into the study, mean rate of decline in the reciprocal of the serum creatinine concentration became less negative. The change predicted simply from regression to the mean was 68.4% of the observed change.(ABSTRACT TRUNCATED AT 250 WORDS)