Objective—To determine size and weight of the pituitary gland and associations between pituitary gland size and weight and sex and age in horses without clinical signs associated with pituitary pars intermedia adenoma (PPIA) and horses and ponies with PPIA. Animals—Pituitary glands from 100 horses without clinical signs of PPIA and 19 horses and 17 ponies with PPIA. Procedures—Pituitary glands were weighed, measured, and examined histologically by use of H&E stain. Masson trichrome and periodic acid-Schiff staining were used, when appropriate. Histologic lesions in the pars intermedia, pars distalis, or both were classified as no significant lesions, single or multiple cysts, focal or multifocal hyperplasia, single or multiple microadenomas, and adenoma. Relative pituitary weight (RPW) was calculated as pituitary weight (grams) divided by body weight (grams). Results—There was an age-related increase in the presence of pituitary lesions in the pars distalis and pars intermedia in geldings, mares overall, and nonpregnant mares. Mean (± SD) RPW in horses with PPIA was not significantly different from ponies with PPIA (15 ± 5.9 X 10–6 and 16 ± 7.2 × 10–6, respectively). Maximum pituitary weight in a horse with PPIA was 13.9 g (RPW, 2.9 × 10–5). Plasma glucose concentration was positively correlated with RPW in ponies with PPIA. Conclusions and Clinical Relevance—Pituitary lesions may be a factor in horses with insulin resistance and laminitis before development of clinical signs of PPIA. Ovarian steroids may be involved in the pathogenesis of lesions in the pars intermedia. (Am J Vet Res 2004;65:1701–1707)