We investigated the agreement between 1) self-reported and technician-measured waist circumference at the level of the umbilicus, 2) circumference measured at the level of the umbilicus and halfway between the lower rib and the iliac crest (the natural waist), and 3) self-reported circumference at the level of the umbilicus and technician-measured circumference at the natural waist. At follow-up in the Danish "Diet, Cancer and Health" study, we recruited 176 men and 240 women for a validation study. Bland-Altman plots were used to evaluate agreement among measurement sites. Multiple regression was used to identify variables explaining the difference between measurements. The participants underestimated their waist circumference; the mean differences were -1.6 cm (95% CI: -2.4 cm, -0.8 cm) in men and -3.0 cm (95% CI: -3.8 cm, -2.3 cm) in women. Limits of agreement were from -11.9 to +8.7 cm among men and -14.9 to +8.9 cm among women. High BMI and large baseline waist circumference were associated with a larger degree of underreporting. Waist circumference measured at the level of the umbilicus was larger than at the natural waist; the mean differences were +0.7 cm (95% CI: +0.4 cm, +1.1 cm) in men and +5.0 cm (95% CI: +4.4 cm, +5.6 cm) in women. The self-reported waist circumference at the level of the umbilicus was correlated with the technician-measured circumference at the natural waist. The circumference at the natural waist was overestimated for women, depending on baseline waist circumference, and slightly underestimated for men, depending on baseline BMI.