Homebound status is a critical eligibility criterion for Medicare reimbursement of some home care services, yet little discussion has been undertaken to establish a valid definition of it. We propose an operational definition of homebound status, and we measure its validity for community- dwelling elderly in the Massachusetts Health Care Panel Study (MHCPS). The MHCPS is a longitudinal study of a cohort of elderly persons (N = 1,625), which began in 1974, with follow-up surveys in 1976, 1980, and 1985. Validity was measured by comparing responses from the operational measure to persons' responses to questions that we judged should be associated with a valid measure of homebound status. This construct validity method resulted in correlations that were significant and in the expected direction, and that suggested that this operational measure is a highly specific, moderately sensitive, valid measure. These results underscore the need for researchers investigating the homebound to discuss the validity and limitations of their homebound measures, and in what context these measures are useful.