Background Menopause is associated with urine phosphorus retention, which is mitigated by estrogen therapy. Fibroblast growth factor 23 (FGF-23) is a hormone originating from bone that regulates urine phosphorus excretion. Whether sex or estrogen therapy is associated with different FGF-23 levels is unknown. Study Design & Setting Cross-sectional study of ambulatory individuals with prevalent cardiovascular disease. Predictors Sex and, in women, use or nonuse of estrogen. Outcomes Serum phosphorus, tubular maximum reabsorption of phosphorus indexed to glomerular filtration rate (TMP/GFR), and plasma FGF-23 concentrations. Results For 987 participants, mean age was 67 ± 11 years, 182 (18%) were women, and 46 (25%) were using estrogen. Mean estimated GFR was 71 ± 23 (SD) mL/min/1.73 m2 . Compared with women who were not using estrogen, both women on estrogen therapy and men had significantly lower serum phosphorus concentrations, lower TMP/GFR values (indicating higher urine phosphorus excretion), and lower FGF-23 concentrations with adjustment for age, demographics, and kidney function ( P < 0.001 for each). Mean FGF-23 levels were 68.7 (95% CI, 59.7-79.0) relative units (RU)/mL in non–estrogen-using women, 43.8 (95% CI, 41.2-46.5) RU/mL in men, and 45.1 (95% CI, 35.2-57.4) RU/mL in women using estrogen in adjusted analysis ( P < 0.001). Limitations Most participants were men. Estrogen therapy was not randomly assigned. Conclusions Older women who are not using estrogen have higher FGF-23 levels than either men or women using estrogen. In the context of prior literature, these data suggest that postmenopausal phosphorus retention may stimulate higher FGF-23 concentrations after menopause.