The utility of multicenter cognitive test methodology and resultant outcomes of supplementation with docosahexaenoic acid in healthy 4-year-old children was evaluated in a randomized, placebo-controlled, double-blind study. Subjects received 400-mg/d docosahexaenoic acid (n = 85) or matching placebo (n = 90) in capsules for 4 months. Cognitive tests included the Leiter-R Test of Sustained Attention, Peabody Picture Vocabulary Test, Day-Night Stroop Test, and Conners' Kiddie Continuous Performance Test. The relationship of docosahexaenoic acid levels in capillary whole blood from a subsample (n = 93) with scores on cognitive tests was evaluated. For each test, results indicated that changes from baseline to end of treatment were not statistically significantly different between the docosahexaenoic acid group and the placebo group. Regression analysis, however, yielded a statistically significant positive (P = .018) association between the blood level of docosahexaenoic acid and higher scores on the Peabody Picture Vocabulary Test, a test of listening comprehension and vocabulary acquisition.