OBJECTIVES: To determine the presence or extent of arginine deficiency in pressure ulcer (PU) patients on percutaneous endoscopic gastrostomy (PEG) feeding and to examine the effects of arginine supplementation on PU healing. DESIGN: All eligible PEG patients, with and without PU, were cross-sectionally assessed for plasma arginine. Three-month supplementation with arginine-enriched water (Arginaid Water) was performed on a subset of patients with PU. This intervention study was a prospective, non-controlled trial with 5 PU patients. SETTING: Geriatric ward of a rural clinical hospital in Japan. PARTICIPANTS: Thirty-nine inpatients with PEG feeding were assessed for plasma arginine. Five of the 13 patients with PU and five of 26 patients without PU underwent amino acid profiling. INTERVENTION: Five of the patients with PU received Arginaid Water supplementation. MEASUREMENTS: Plasma amino acid measurements and biochemical analyses were performed. For those with PU on Arginaid Water supplementation, plasma arginine concentration and PU status were monitored every month. RESULTS: Patients with PU showed significantly lower plasma arginine concentration compared to those without PU (control vs. PU; 80.2±21.3 vs 62.8±14.7 nmol/ml, p<0.01). After the addition of Arginaid Water, plasma arginine concentration increased (before vs 3 months later; 57.9±1.8 vs 83.1±8.5, p<0.01), and PU area, perimeter, DESIGN-R and PUSH scores significantly improved. CONCLUSION: Plasma arginine was lower in PEG patients with PU. The healing rate of PU is improved with Arginaid Water supplementation. The findings from this study support the use of arginine supplementation in PEG patients with PU.