This property, which is tainted by default, is an integer relating to the currently-selected option of a Select object. If, however, the Select object allows for multiple selections (i.e. when the <SELECT> tag includes the MULTIPLE attribute), the selectedIndex property will only return the index of the first option selected.
document.myForm.mySelect.selectedIndex
For example, this code would return the index of the selected option of a Select object called MySelect in MyForm.