The results of NIOSH heat stress evaluations utilizing a new personal heat stress monitor were summarized. The earplug monitor measured body temperature using a small sensor that was inserted in the ear canal. The monitor contained an audible alert system that was activated if the body temperature exceeded an adjustable preset level of 38 degrees-C. A second sensor was used to measure ambient temperature. Heat stress evaluations involved workers in a textile dyeing, printing, and finishing factory, a company that produced aircraft fuel cells and portable storage tanks, and a wholesale bakery, as well as steam tunnel workers. Parallel area heat stress monitoring was also conducted using a wet bulb global temperature (WBGT) monitor. Metabolic heat production rates were estimated from the WBGT data and by observation of working positions and activities. The ear canal and WBGT monitors indicated that significant heat stress occurred to workers at all worksites except in the bakery. The ear canal monitor was repeatedly activated in the bakery workers, although ambient WBGT monitoring indicated that high heat stress conditions did not exist. Estimates of metabolic heat production indicated that heat stress had not occurred. Rectal temperatures of the bakery workers indicated a maximum body temperature of only 37.5 degrees. The discrepancy was attributed to the ear canal monitors being calibrated in an air conditioned office before being used in the bakery. The author concludes that personal heat stress monitors can be effectively used to evaluate workers ability to work in hot environments and to evaluate the relative heat stress associated with specific tasks. Heat stress monitoring should be an essential component of any heat stress program.