Dietary factors affect urine pH. Alkaline urine is observed in persons who eat large quantities of citrus fruit and vegetables. Acid urine is observed with high meat intake. Pyridium® metabolites may mask the pH reaction. Urine pH >6.5 indicates presence of bicarbonate while pH <5.5 indicates absence of bicarbonate. Consistently acid urine, pH <5.5, is associated with xanthine, cystine, and uric acid stones. Calcium oxalate and apatite stones are not associated with any particular disturbance of urine pH. Alkaline urine (pH >7) is associated with calcium carbonate, calcium phosphate, and especially magnesium ammonium phosphate stones. In conjunction with serum pH and bicarbonate levels, urine pH may be applied to the study of renal tubular acidification.