Proteinuria has been shown to be strongly associated with the prevalence and incidence of cardiovascular disease. It has been difficult to determine if the link is causal and independent. The mortality follow-up for the Multiple Risk Factor Intervention Trial (MRFIT) randomized cohort provides an opportunity to examine these relationships. Between 1973 and 1975, 361,662 men, ages 35 to 57, were screened for blood pressure, serum cholesterol, and cigarette smoking. Patients receiving medication for diabetes were excluded. Men in the upper 10 to 15% of coronary heart disease (CHD) risk (12,866) were randomized into the MRFIT trial. Standard casual urine dipstick determinations (Labstix) for protein were done at baseline and annually for six years. Post-trial cause-specific mortality was ascertained using the National Death Index. During the trial, 2326 (18.1%) of participants had + or higher proteinuria, and 593 (4.6%) had +2 or higher proteinuria. The presence of proteinuria during the six years of follow-up was consistently associated with higher all cause, cardiovascular disease (CVD) and CHD mortality, even after adjusting for other risk factors. The higher and more persistent the proteinuria, the greater the risk. In this data set, proteinuria is a strong and independent risk factor for CVD mortality.