With an increasing number of older adult online users, the Internet has become recognized as an efficient medium to disseminate health information to these individuals. Usability of health Web sites is particularly important for older adults who have unique Web design needs. Although several Web usability questionnaires have been developed for younger adults, there is a lack of such measure for older adults. In this study, based on prior research findings and consultation with usability experts, a 12-item Perceived Health Web Site Usability Questionnaire (PHWSUQ) was developed for older adults. The preliminary reliability and validity of this measure was assessed in a study that tested the usability of three health Web sites employing 10 older adult online users. The findings provided some evidence of internal consistency and construct validity for the PHWSUQ. The major limitation of this study was its small sample size, hence further psychometric testing is needed with larger samples.