Giardia lamblia infection was identified in 33 of 89 (37%) 3-month-old to 3-yr-old children who were followed with monthly stool examinations for up to 12 months in a day care center. The infection was mainly asymptomatic and usually associated with prolonged carriage of the parasite. There were no significant differences for height and weight achievements and mean hemoglobin values between Giardia-positive and Giardia-negative children. However, Giardia-positive children tended to achieve higher weight and height for age than Giardia-negative children; weight for age was above the 50th percentile in 69% of Giardia-positive vs. 40% of Giardia-negative children (alpha = 0.01). Giardia-positive children tended to have fewer symptoms related to the gastrointestinal and respiratory tracts as recorded by a weekly questionnaire. Lactase deficiency was detected by breath hydrogen testing in 8 of 26 Giardia-positive vs. only 1 of 21 Giardia-negative children (P less than 0.02). Healthy day care children with asymptomatic Giardia infection show no disadvantage and perhaps even an advantage in nutritional status and freedom from other illnesses.