Residual renal function (RRF) is a key element for good maintenance of patients on peritoneal dialysis (PD). Several reports have been published comparing RRF preservation between continuous ambulatory PD and automated PD (APD); however, no comparisons have yet been made between tidal and non tidal APD. We therefore retrospectively analyzed RRF in patients on tidal (n = 10) and non tidal (n = 19) APD and tried to elucidate factors that differed between them. We observed no statistical differences in background, body mass index, RRF urinary volume, peritoneal clearance, ultrafiltration (UF) volume, daily infusion volume, creatinine generation rate (CGR), or urea generation rate between the two groups at that start of PD. However, after 3 years, renal creatinine clearance (CCr) and urinary output in the tidal group were statistically higher than those in the non tidal group. Conversely, UF and peritoneal CCr were lower. Although the CGR in the tidal group increased, it decreased in the non tidal group, leading to a statistically significant difference after 3 years. The dialysate-to-plasma creatinine and total CCr showed no statistical difference between the two groups. These results indicate that RRF is better preserved in tidal than in non tidal APD.