The relationship between renal metabolism and urinary acidification is poorly understood. During the past decade evidence has accrued to suggest that the hexose monophosphate (HMP) shunt might serve in the process of urinary acidification by providing reducing equivalents for a redox-coupled membrane-bound proton pump that could transport protons into the tubular lumen. The major support for this hypothesis has come from the finding that HMP shunt activity increases with acute and chronic metabolic acidosis. In the present study, we examine the urinary acidification capacity of a young man with severe erythrocyte glucose-6-phosphate dehydrogenase (G-6-PD) deficiency and with unmeasurable G-6-PD activity in renal cortical tissue. We found that despite unmeasurable G-6-PD activity in renal tissue, the patient was capable of generating a maximally acid urine and increasing total acid secretion. Our findings suggest that the HMP shunt may not be necessary for the urinary acidification process. © 1980.