Obesity increases cancer risk, yet small-scale surveys indicate that obese women delay or avoid cancer screening even more so than do nonobese women. We sought to estimate the association between body mass index (BMI) (kg/m(2)) and delayed cancer screening among adult women in a population-based survey. Subjects were women classified by BMI as underweight (<18.5), desirable weight (18.5-24.9), overweight (25-29.9), obese class I (30-34.9), obese class II (35-39.9), and obese class III (> or =40). Outcome measures were intervals (0 for < or =2 years versus 1 for >2 years) since most recent screening for Papanicolaou (Pap) smear, mammography, and clinical breast examination (CBE). Adjusting for age, race, smoking, and health insurance, we observed J-shaped associations between BMI and screening. Compared with desirable weight women, underweight women (odds ratios [OR] = 1.21, 95% confidence interval [95% CI] 1.09-1.34), overweight women (OR = 1.13, 95% CI 1.07-1.18), and obese women (OR range 1.22-1.69) were significantly more likely to delay Pap smear testing for >2 years. Underweight (OR = 1.32, 95% CI 1.13-1.54), obesity class I (OR = 1.12, 95% CI 1.02-1.23), and obesity class III women (OR = 1.32, 95% CI 1.10-1.54) were more likely to delay mammography, and overweight (OR = 1.10, 95% CI 1.01-1.19), obesity class I (OR = 1.18, 95% CI 1.08-1.30), and obesity class III women (OR = 1.47, 95% CI 1.23-1.75) were more likely to delay CBE. White women were more likely to delay CBE as a function of BMI than were non-white women. Weight may be an important correlate of cancer screening behavior, particularly for white women.