BACKGROUND House dust mites, Dermatophagoides farinae and Dermatophagoides pteronyssinus, are important components in the development of asthma. OBJECTIVE We measured the prevalence of house dust mites in a tertiary care hospital located in a temperate geographic region where dust mites were prevalent in homes. METHODS The density of Dermatophagoides spp. was measured in hospital hallways, carpeted patients' rooms, and noncarpeted patients' rooms by vacuuming the floor in the summer and winter seasons. Bedrooms in the homes of employees were sampled as a summer control group. A total of 141 dust samples was obtained. RESULTS No D. farinae or D. pteronyssinus was found in 60 hospital dust samples that were obtained during the winter season. Although mites were found in some locations in the hospital during the summer dust collection, mite density in these locations and the average mite density for all samples were insignificant. During the summer dust samples from the bedroom carpets of all employees' homes sampled were positive for mites, with many homes having moderate or high populations (range, 22 to 8340 mites per gram of dust). CONCLUSIONS The dust mite prevalence in a hospital could be kept very low even though mite levels in employees' homes were moderate to high. The factors responsible for the low mite density in the hospital were maintenance of low relative humidity, use of low-pile carpets, and good housekeeping and laundering practices.