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Extreme summertime conditions result in millions of dollars in lost revenue every year due to production and death losses. Anecdotal evidence suggests that these losses stem from animals particularly vulnerable to heat stress. A study was conducted to determine risk factors for heat stress in feedlot heifers. Two-hundred fifty-six feedlot heifers of four genotypes (32/genotype/year) were observed for 6 8 weeks in two consecutive summertime periods. As a measure of stress, respiration rates and panting scores were taken twice daily (morning and afternoon) on a random sample of 10 heifers/genotype. Weights, condition scores, and temperament scores were taken on a 28-day interval during the experiment. Health history from birth to slaughter was available for every animal used in this study. It was determined that stress level (respiration rate/ panting score) was impacted by genotype (or color), a history of respiration illness, temperament, and degree of fatness. Daily weight gains were also impacted by genotype (or color), a history of respiration illness, and temperament. These results illustrate the sensitivity of respiration rate as an indicator of stress, and indicate that animals at risk for heat stress can be identified for possible applications of precision animal management.