Option lists with many items may cause problems for users with assistive technology, such as screen readers. The solution is to use the Optgroup element to split options in the Select list into manageable groups. This also better practice for usability.
WCAG 1.0 says: Guideline 2.3 Divide large blocks of information into more manageable groups where natural and appropriate. [Priority 2]
- The SELECT, OPTGROUP, and OPTION elements (W3C)
- Grouping Form Controls (W3C)
- Accessibility Conformance Tests, Further Improvements
- Accessible Forms from HTML Dog