Residual renal function (RRF) is an important factor in the well-being of peritoneal dialysis (PD) patients. Serum phosphate has been correlated with long-term morbidity and mortality. We wished to determine if RRF contributes to a lower level of serum phosphate and magnesium. We also investigated the relationship between protein catabolic rate (PCR) and phosphate and magnesium. We collected data related to serum phosphate, serum magnesium, PCR, and RRF in 100 PD patients. The Pearson correlation coefficient was used to study the correlation between RRF and magnesium, RRF and phosphate, PCR and magnesium, and PCR and phosphate. No relationship were seen between PCR and serum phosphate, and RRF and serum magnesium. Significant relationships were seen between PCR and serum magnesium and phosphate. The very good inverse correlation between RRF and serum phosphate highlights the importance of RRF in the control of serum phosphate. A very good relationship was also seen between PCR and serum magnesium.