['On the basis of experimental chamber studies, vacuum cleaners with double-thickness bags and integral high-efficiency particulate arrest (HEPA) air filters are claimed to reduce airborne allergen levels and are currently recommended to allergic patients.', 'The objective of this study was to investigate the effect of vacuum cleaning on personal inhaled cat allergen exposure in homes with cats.', 'Five unused new vacuum cleaners were compared with an old non-HEPA filter vacuum cleaner. Each vacuum cleaner was tested in an experimental chamber and in 5 homes with cats. Inhaled cat allergen was measured by nasal air sampling.', 'New vacuum cleaners failed to leak any allergen in the experimental chamber. There was a significant increase in inhaled cat allergen during vacuum cleaning in homes (F = 48.39, df = 1.4, P =.002) with no difference between the old vacuum cleaner and the unused new vacuum cleaners (5-fold and 3-fold increase compared to baseline, respectively; F = 0.005, df = 1.4, P =.95).'] Do high-efficiency particulate arrest-filter vacuum cleaners increase personal cat allergen exposure in homes with cats?