Chronic health problems may be related to body mass index (BMI, kg/m(2)), but this has been best documented in overweight and obese adults. The primary objective of this study was to identify factors associated with different categories of BMI in elderly men and women from the general population, also including the lower categories of BMI. In a cross-sectional population survey from the municipality of Tromsø, Norway we analyzed associations between BMI and a wide range of chronic disease conditions, lifestyle and socioeconomic factors. BMI was categorized into six groups (<20, 20.0-22.4, 22.5-24.9, 25.0-27.4, 27.5-29.9, > or =30.0 kg/m(2)). The study included 4,259 men and women aged 65 years and older from the general population. We found low relative weight (BMI < 20 kg/m(2)) to be associated with increasing prevalence of mental distress, hip fracture, smoking and low handgrip strength. A U-shaped relation to BMI was found for asthma and chronic bronchitis, poor current health and low physical activity. The higher categories of BMI were associated with low education level, a difficult economical situation, diabetes mellitus and ischemic heart disease. These results demonstrate that both low and high BMI are associated with a wide range of prevalent conditions and diseases in elderly men and women. For the clinician the findings emphasize the importance of nutritional assessment as part of the medical evaluation of elderly patients.