Renal failure is common among the long-term care (LTC) elderly. Little is known about the acid/base status of these patients. The aim of this study is to evaluate the relationship between the acid base status and renal function in a representative group of skilled nursing patients and relate it to their feeding status. LTC elderly patients, in stable clinical condition, 50 on naso-gastric tube (NGT) feeding, 40 orally fed (OF), were recruited to this study. As controls, we studied a group of 30 elderly independent, ambulatory patients admitted to the acute geriatric departments of the hospital for different causes which were not related to their acid-base status. Venous blood was taken for the routine tests and blood gases. In the LTC study groups a 24-h urine collection was examined for biochemical parameters and calculations of all clearances. Glomerular filtration rate (GFR) was estimated by the Cockroft and Goult and MDRD formulas. Renal function was similar in the two main study groups. Daily secretion of sodium and chloride were 50% lower in the NGT fed patients (p<0.001). The LTC elderly patients had significantly higher venous pH values, with no differences in pCO(2) or HCO(3). An alkalotic state (pH>7.45) was found in 13.6% of them (18% in the NGT and 6.5% in the OF) while none of the independent elderly had such values (p<0.05). Similarly, HCO(3)>34 was found in 12% of the LTC elderly versus none in the independents (p=0.06). Values of pO(2) and O(2) saturation were significantly higher in the nursing elderly and mainly those fed by NGT. Hemoglobin levels had a significantly negative correlation with the pH (r=-0.3, p<0.002). In conclusion, unexpected metabolic alkalosis was found in a group of skilled nursing patients, more prominent in those fed by NGT. This finding warrants the inclusion of routine pH determination in patients whenever pharmacokinetic considerations are essential.