Objective: To examine patterns of the incidence of diabetes, hypertension, and mortality by single units of body mass index (BMI) and 5-year age groups using longitudinal data from middle-aged and older women. Patients and Methods: Middle-aged (born between 1946 and 1951; N=13,715) and older (born between 1921 and 1926; N=12,432) participants in the Australian Longitudinal Study on Women's Health completed surveys in 1996 and at approximately 3-year intervals thereafter until 2011. Proportions of women with diabetes, hypertension, and mortality over 3-year intervals were estimated for each unit of BMI and 5-year age group (45 to <50, 50 to <55, 55 to <60, 70 to <75, 75 to <80, and 80 to <85 years) using generalized additive modeling with adjustment for time-varying covariates. Results: Three-year incidence of diabetes (1.2%-3.6%), hypertension (5.2%-17.8%), and death (0.4%-9.5%) increased with age. For both diabetes and hypertension, the associations with BMI were curvilinear in middle-aged women and became almost linear in older women. With increasing age, the slope became steeper, and the increase started at lower BMI values. For hypertension, there was a marked increase in intercept from 75 years onward. In contrast, mortality risks were highest for low BMI (≤20) in all age groups. A clear U-shaped curve was observed only in the oldest age group. Conclusion: The shapes of the relationships between BMI and incidence of diabetes, hypertension, and mortality change with age, suggesting that weight management interventions should be tailored for different age groups.