Prevalence and intensity of Ascaris lumbricoides infection were analyzed by a cross-sectional study in children addressed in the city of Duque de Caxias, Rio de Janeiro State, from July to December 1997. The study population consisted of children under two years of age (n = 387) and mothers of children under one year of age (n = 104). Coproparasitologic tests were run using the MIFC and the Kato thick smear (Kato-Katz) methods. Prevalence was 3.3% (95% CI: 1.0-7.8) for children under one year, 30.7% (95% CI: 25.4-36.4) for one-year-olds, and 42.3% (95% CI: 33.0-51.9) for mothers of children under one. Some 38.0% of children under one year of age showed medium-to-heavy infection. These results indicated that ascariasis was an important problem in the first year of life and demonstrated the need for research in the area of maternal and infantile ascariasis.