Summaryobjective To investigate the relationship between hookworm and Ascaris lumbricoides infection and performance on three subsets of the Wechsler Intelligence Scale for Children -third edition (WISC-III) (Digit Span, Arithmetic and Coding) and Raven Colored Progressive Matrices.methods Cross-sectional study of 210 children between the ages of 6 and 11 years in Americaninhas, Minas Gerais, Brazil. Separate proportional odds models were used to measure the association between the intensity of helminth infections and poor performance on each of the four cognitive tests.results After adjusting for sex, age, socioeconomic status and other helminth infections, moderate-tohigh-intensity hookworm infection was associated with poor performance on the WISC-III Coding subtest [OR = 3.20; 95% confidence interval (CI) = 1.43-7.17], low intensity of hookworm infection was associated with poor performance on the WISC-III Coding subtest [odds ratio (OR) = 3.71; 95% CI = 1.80-7.66] and moderate-to-high-intensity A. lumbricoides infection was associated with poor performance on the Raven test (OR = 2.03; 95% CI = 1.04-3.99), all in comparison with uninfected children. Children co-infected with A. lumbricoides infection and hookworm infection had greater odds of poor performance on some WISC-III subtests than children with only A. lumbricoides infection.conclusions These findings suggest that hookworm infection may be associated with poorer concentration and information processing skills, as measured on the WISC-III Coding subtest, and that A. lumbricoides infection may be associated with poorer general intelligence, as measured through the Raven Colored Progressive Matrices. This study also presents evidence that polyparasitized children experience worse cognitive outcomes than children with only one helminth infection.