The effect of a low protein intake on survival in renal transplant recipients (RTR) is unknown. A low protein intake may increase risks of malnutrition, low muscle mass, and death. We aimed to study associations of protein intake with mortality and graft failure and to identify potential intermediate factors. Protein intake was estimated from 24-h urinary urea excretion (24-h UUE). Graft failure was defined as return to dialysis or retransplantation. We used Cox regression analyses to analyze associations with outcome and potential intermediate factors in the causal path. In 604 RTR, mean +/- SD 24-h UUE was 380 +/- 114mmol/24-h. During median follow-up for 7.0yr (interquartile range: 6.2-7.5yr), 133 RTR died and 53 developed graft failure. In univariate analyses, 24-h UUE was associated with lower risk of mortality (HR [95% CI]=0.80 [0.69-0.94]) and graft failure (HR [95% CI]=0.72 [0.56-0.92]). These associations were independent of potential confounders. In causal path analyses, the association of 24-h UUE with mortality disappeared after adjustment for muscle mass. Low protein intake is associated with increased risk of mortality and graft failure in RTR. Causal path analyses reveal that the association with mortality is explained by low muscle mass. These findings suggest that protein intake restriction should not be advised to RTR.