Item response theory was used to address gender bias in interest measurement. Differential item functioning (DIF) technique, SIBTEST and DIMTEST for dimensionality, were applied to the items of the six General Occupational Theme (GOT) and 25 Basic Interest (BI) scales in the Strong Interest Inventory. A sample of 1860 women and 1105 men was used. The scales were not unidimensional and contain both primary and minor dimensions. Gender-related DIF was detected in two-thirds of the items. Item type (i.e., occupations, activities, school subjects, types of people) did not differ in DIF. A sex-type dimension was found to influence the responses of men and women differently. When the biased items were removed from the GOT scales, gender differences favoring men were reduced in the R and I scales but gender differences favoring women remained in the A and S scales. Implications for the development, validation and use of interest measures are discussed. (Contains 4 figures and 7 tables.)