The objective of this study was to examine the factors associated with perceived and actual morbidity among older adults during the August 2003 heat wave within two cohorts: PAQUID and Trois Cités (3C). Data were collected on variables related to the subjects’ and their housing’s characteristics, health status, coping behaviors during the heatwave, use of healthcare services, and the occurrence of fatal and morbid events during the heatwave. The PAQUID cohort study included 879 participants aged 80 and older, of whom 11% experienced an actual impact from the heat and 12% a subjective worsening of their health status during the heatwave. The absence of a bathroom in the home, the presence of a chronic condition (respiratory, neurological, cardiovascular, or sleep and anxiety disorders), and regular use of healthcare services were factors associated with a higher risk of experiencing the impact of the heat wave. Among the 1,416 individuals aged 70 and older in the 3C cohort, 6% experienced a real impact and 7% reported a worsening of their health during the heat wave. Asthma, depression, and living alone were associated with a higher risk of suffering from the heat wave. In both cohorts, participants who stopped their activities during the heatwave or frequently sought care from healthcare professionals had a higher risk of being affected by the heat; conversely, those who were able to ventilate their homes were at lower risk. In conclusion, simple measures such as ventilating the home or wearing appropriate clothing can help reduce heat-related morbidity among older adults. Furthermore, the risk appears to be higher among individuals who are dependent or compromised by a chronic condition, suggesting the need for enhanced monitoring of these individuals. (R.A.)