A convenience sample of 501 children 7 to 13 years old completed the newly developed Health Self-Determinism Index for Children (HSDI-C), a measure of intrinsic motivation in health behavior. Construct validity was supported through the factorial isolation of four theory-consistent subscales, and the correlation of the total HSDI-C (r=.36, p ≤ .009) with a general measure of intrinsic motivation. Alpha coefficients in three separate trials ranged between .79 and .88 for the total scale, and .78 to .88, .87 to .92, .78 to .86, and .63 to .77 on the respective subscales. Test-retest reliability for the total scale was .83 at 2 weeks and .48 at 1 year. A nominated population of 21 third- through seventh-grade children known to practice positive health promotion behaviors had an HSDI-C mean score of 106.5 versus 76.5 in the general sample of children.