OBJECTIVE To determine whether antepartum pyelonephritis causes an acute or delayed alteration in erythropoietin production. METHODS Serum erythropoietin concentrations were determined prospectively using an enzyme-linked immunosorbent technique in 36 pregnant women admitted to Parkland Hospital with pyelonephritis. Healthy nonanemic pregnant women served as controls. RESULTS Serum erythropoietin levels in women with antepartum pyelonephritis were not different from those measured in normal pregnant women. Specifically, there were no differences in erythropoietin levels in women who had anemia at admission (n = 6; 13.8 mU/mL), hemolysis (15.4 versus 12.9 mU/mL), or renal insufficiency (14.5 versus 12.9 mU/mL) secondary to renal infection as compared to controls. CONCLUSIONS We conclude that antepartum pyelonephritis does not alter erythropoietin production either acutely or within several days of infection. Because erythropoietin production was not decreased, we suggest that hemolysis is the major factor contributing to anemia associated with renal infection.