Low numeracy skills and obesity are both common. Numeracy skills are used in healthy weight management to monitor caloric intake. The relationship between obesity and numeracy skills in adult primary care patients is unknown. A cross-sectional study enrolled adult, English-speaking primary care patients. BMI was assessed by self-report; numeracy and literacy skills were measured with the Wide Range Achievement Test, 3rd Edition (WRAT-3) and the Rapid Estimate of Adult Literacy in Medicine (REALM), respectively. The relationship between numeracy and BMI was described with Spearman's rank correlation and linear regression analyses. In 160 patients, the mean (s.d.) age was 46 (16) years, 66% were white, 70% were female, and 91% completed high school. The mean BMI was 30.5 (8.3) kg/m(2). Less than 9th grade numeracy skills were found in 66% of the participants. Participants with numeracy skills <9th grade had a mean BMI of 31.8 (9.0) whereas those with numeracy skills > or =9th grade had a mean BMI of 27.9 (6.0), P = 0.008. Numeracy was negatively and significantly correlated with BMI (rho = -0.26, P = 0.001). This correlation persisted after adjusting for age, sex, race, income, years of education, and literacy (beta coefficient = -0.14; P = 0.010). Literacy skills were not associated with BMI. We found a significant association between low numeracy skills and higher BMI in adult primary care patients. A causal relationship cannot be determined. However, numeracy may have important clinical implications in the design and implementation of healthy weight management interventions and should be further evaluated to determine the magnitude of its effect.