In the USA, infant formulas contain long-chain PUFA arachidonic acid (ARA) and DHA in a ratio of 2:1 and comprise roughly 0·66 g/100 g and 0·33 g/100 g total fatty acids (FA). Higher levels of dietary DHA appear to provide some advantages in visual or cognitive performance. The present study evaluated the effect of physiologically high dietary ARA on growth, clinical chemistry, haematology and immune function when DHA is 1·0 g/100 g total FA. On day 3 of age, formula-reared (FR) piglets were matched for weight and assigned to one of six milk replacer formulas. Diets varied in the ratio of ARA:DHA as follows (g/100 g FA/FA): A1, 0·1/1·0; A2, 0·53/1·0; A3-D3, 0·69/1·0; A4, 1·1/1·0; D2, 0·67/0·62; D1, 0·66/0·33. A seventh group was maternal-reared (MR) and remained with the dam during the study. Blood collection and body weight measurements were performed weekly, and piglets were killed on day 28 of age. No significant differences were found among any of the FR groups for formula intake, growth, clinical chemistry, haematology or immune status measurements. A few differences in clinical chemistry, haematology and immune function parameters between the MR pigs and the FR groups probably reflected a difference in growth rate. We conclude that the dietary ARA level up to 1·0 g/100 g total FA is safe and has no adverse effect on any of the safety outcomes measured, and confirm that DHA has no adverse effect when ARA is at 0·66 g/100 g FA.