Abstract Some horses are more prone to obesity than others. This study aimed to compare energy intake and expenditure between obese and lean horses. Ten mature, stock-type geldings (aged 11.8 ± 4.0 years; 577 ± 52 kg) maintained on the same pasture were classified as obese (n = 6, body condition score [BCS] ≥ 7) or lean (n = 4, BCS = 4–5) and adapted to free-choice grass hay over 25 days. Dry matter intake (DMI) was measured for 11 days, with body weight (BW) and blood samples (analyzed for serum leptin) collected on Days 0, 2, 4, 7, 9, and 11. Behavior was observed for 24 hours. Apparent dry matter digestibility and gross energy digestibility were measured using 48-hour total fecal collection. Digestible energy intake was calculated. Activity (distance traveled, average heart rate [HR], and maximum HR) was quantified using HR monitors with GPS tracking. Average HR was used to estimate energy expenditure. Serum leptin, DMI, and BW were analyzed as repeated measures. Other variables were analyzed using unpaired t-tests. P