The lipid chemistry of the normal equine hoof, together with the effect of oral supplementation with an evening primrose oil mixture (EPOM) on its growth, growth rate and lipid content was assessed in a controlled and blinded feeding trial at the Defence Animal Centre. Twelve horses were paired as closely as possible according to sex, age, weight, height and colour and then one from each pair was randomly allocated to treatment or control groups. The treatment group received 30 ml of oral EPOM/day, otherwise the nutrition and management regimes were the same for all horses. No significant differences (P>0.05) were seen between treatment and control groups for hoof horn growth or growth rate. However, there was a significant difference (P<0.05) in hoof horn growth within the treatment group only between weeks 4 and 8 after the start of supplementation. The stratum medium contained significantly higher amounts of cholesterol ester (P<0.05), triglycerides (P<0.001) and free fatty acids (P<0.05) than the periople. The periople contained significantly higher levels of free cholesterol and phospholipid (P<0.001) than the stratum medium of the hoof wall. There were no significant differences (P>0.05) between treatment and control groups for any of the lipid fractions measured for the stratum medium from the clippings of the hoof wall. However, there were differences in perioplic lipid analysis with significant increases (P<0.05) in cholesterol esters and partial glycerides and a significant reduction (P<0.001) in free cholesterol in the treatment group following supplementation.