['To assess the influence of the body mass index (BMI) on the prevalence and severity of chronic daily headache (CDH) and its most frequent subtypes, transformed migraine (TM) and chronic tension-type headache (CTTH).', 'The authors gathered information on headache, medical features, height, and weight using a computer-assisted telephone interview. Participants were divided into five categories, based on BMI: underweight (<18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9), obese (30 to 34.9), and morbidly obese (>35). The prevalence and severity of CDH, TM, and CTTH were assessed. Multivariate analyses modeling these diagnoses as a function of BMI were conducted.', 'Among 30,215 participants, the prevalence of CDH was 4.1%; 1.3% had TM and 2.8% CTTH. In contrast with the normal weight group (3.9%), the prevalence of CDH was higher in obese (5.0% [odds ratio (OR) = 1.3, 95% CI = 1.1-1.6]) and morbidly obese (6.8% [OR = 1.8, 95% CI = 1.4 to 2.2]). BMI had a strong influence on the prevalence of TM, which ranged from 0.9% of the normal weighted to 1.2% of the overweight (OR = 1.4 [1.1 to 1.8]), 1.6% of the obese (OR = 1.7 [1.2 to 2.43]), and 2.5% of the morbidly obese (OR = 2.2 [1.5 to 3.2]). The effects of the BMI on the prevalence of CTTH were just significant in the morbidly obese group. Adjusted analyses showed that obesity was associated with CDH and TM but not CTTH.'] Is obesity a risk factor for transformed migraine but not chronic tension-type headache?