Our objective was to determine if feeding a high-fat supplement versus a control supplement to growing yearling mares would affect growth and/or body composition parameters as assessed via body weight (BW), body condition scores (BCS) and concentrations of plasma hormones. Eight yearling mares were paired by initial BW (348±19 kg) and maintained on pasture and supplemented with either a high-fat supplement (16% fat) or a control supplement (3% fat) at 0.8% of their BW in two daily meals for 8 weeks. Both BW and BCS increased for all mares throughout the study (each P < 0.0001); however, no difference in BW or BCS could be attributed to treatment effects. Nonetheless, plasma concentrations of leptin were greater in mares fed the high-fat supplement (P = 0.0001) compared with the control supplement. Plasma concentrations of growth hormone tended to be greater in high-fat-fed mares (P = 0.06). Plasma concentrations of insulin did not differ between treatment groups (P = 0.96). Although no gross difference in BW or BCS was discernable among mares fed the control versus high-fat supplement, these data provide evidence that increasing fat content in the diet may alter leptin levels independent of changes in body composition.