OBJECTIVE: To examine relationships of BMI with health-related quality of life in adults 65 years and older. RESEARCH METHODS AND PROCEDURES: In 1996, a health survey was mailed to all surviving participants > or = 65 years old from the Chicago Heart Association Detection Project in Industry Study (1967 to 1973). The response rate was 60%, and the sample included 3981 male and 3099 female respondents. BMI (kilograms per meter squared) was classified into four groups: underweight (<18.5), normal weight (18.5 to 24.9), overweight (25.0 to 29.9), and obese (> or = 30.0). Main outcome measures were Health Status Questionnaire-12 scores (ranging from 0 to 100) assessing eight domains: health perception, physical functioning, role limitations-physical, bodily pain, energy/fatigue, social functioning, role limitations-mental, and mental health. The higher the score, the better the outcome. RESULTS: With adjustment for age, race, education, smoking, and alcohol intake, obesity was associated with lower health perception and poorer physical and social functioning (women only) but not impaired mental health. Overweight was associated with impaired physical well-being among women only. Both underweight men and women reported impairment in physical, social, and mental well-being. For example, multivariable-adjusted health perception domain scores for women were 50.8 (underweight), 62.7 (normal weight), 60.5 (overweight), and 52.1 (obese), respectively. Associations weakened but remained significant with further adjustment for comorbidities. DISCUSSION: Compared with normal-weight people, both underweight and obese older adults reported impaired quality of life, particularly worse physical functioning and physical well-being. These results reinforce the importance of normal body weight in older age.