It has been reported that boron may be beneficial for optimal calcium metabolism and, thus, optimal bone metabolism. Therefore, we designed a study to determine the effects of boron supplementation on blood and urinary minerals in athletic subjects and sedentary control subjects consuming self-selected typical Western diets. Serum phosphorus concentrations were lower in boron-supplemented subjects than in placebo-supplemented subjects. Compared with all other subjects, serum magnesium concentrations were greatest in the sedentary control subjects supplemented with boron and increased with time in all subjects. Exercise training diminished changes in serum phosphorus concentrations caused by boron supplementation. Calcium excretion increased over time in all groups combined, and boron excretion increased over time in all boron-supplemented subjects. The findings suggest that boron supplementation modestly affected mineral status, and exercise modified the effects of boron supplementation on serum minerals.