OBJECTIVES: To identify factors associated with the increase in the use of bilevel positive pressure ventilators with timed backup in the Medicare population from 1996 through 1999. METHODS: We extracted durable medical equipment claims for bilevel positive pressure ventilators with and without timed backup (RAD and RAD-timed, respectively), continuous positive airway pressure devices (CPAP) and oxygen supplies (gas, liquid, concentrator) from the 1994–1999 claims files for 5% of Medicare beneficiaries, and compared disease prevalence, oxygen use, and other factors in the different user groups. RESULTS: Between 1994 and 1998, new use increased for both RAD and RAD-timed devices, but was significantly greater for the latter. Oxygen use remained relatively stable among CPAP and RAD users, but increased in RAD-timed users. RAD-timed users were more likely than RAD users to have claims for oxygen (80% vs. 44%), or diagnoses suggesting chronic lung disease (86% vs. 35%) in the first year of device use; RAD users were more likely to have a diagnosis of sleep apnea/other sleep disorder in the first year of use (73% vs. 14%). CONCLUSIONS: Debate continues over appropriate indications, servicing and reimbursement for respiratory assist devices, particularly those with timed backup. RAD and RAD-timed device users differ significantly. The high prevalence of COPD diagnoses reflects current interest in the use of these devices in the long-term treatment of this disorder; other indications suggested in the literature, such as respiratory support in neuromuscular disorders or congestive heart failure, appeared rarely. Growth in the use of the more costly RAD-timed devices among oxygen users varied by region as well as by year, possibly reflecting regional differences in coverage, medical practice or marketing. Person-level, longitudinal analysis of claims data supports the development and implementation of coverage and payment policies, by helping to delineate the demographic and clinical characteristics of user groups.