The development of preferential looking acuity was studied prospectively to 3 mo of age in exclusively breast-fed and formula-fed term gestation infants. The formula contained (% of total fatty acids) 17.9% linoleic acid (18:2 omega-6) and 2.1% alpha-linolenic acid (18:3 omega-3) but no docosahexaenoic acid (22:6 omega-3) or arachidonic acid (20:4 omega-6). The breast milk contained (mean +/- SEM) 13.4 +/- 0.8% 18:2 omega-6, 1.5 +/- 0.1% 18:3 omega-3, 0.51 +/- 0.03% 20:4 omega-6, and 0.22 +/- 0.02% 22:6 omega-3. Preferential looking acuity, assessed by the acuity-card procedure, and plasma phospholipid and erythrocyte phosphatidylcholine and phosphatidylethanolamine fatty acids were determined at 14 d and 3 mo of age. There were no significant differences in acuity at 14 d or 3 mo, despite substantial differences in erythrocyte and plasma lipid 22:6 omega-3. Visual acuity was [mean (cycles/degree) +/- SD (octaves)] 3.93 +/- 0.54 and 4.77 +/- 0.48 and erythrocyte phosphatidylethanolamine %22:6 omega-3 was (mean +/- SE) 7.6 +/- 0.5 and 4.0 +/- 0.2 in the 3-mo-old breast-fed and formula-fed infants, respectively. These studies show that feeding formula containing 2.1% 18:3 omega-3 (approximately 1.0% energy) results in development of visual acuity similar to breast-feeding in term infants to > or = 3 mo of age.