The records of 21 horses with an adenoma of the pars intermedia of the pituitary gland were reviewed. The animals comprised nine Dutch warmblood horses and 12 ponies. They ranged in age from 12 to 30 years (mean +/- sem 21 +/- 1.3 years) for 20 of the horses, and comprised 13 geldings and eight mares. All the animals showed hirsutism. In 1991, nine horses were diagnosed as having the adenoma, equivalent to 0.5 per cent of the horses examined in 1991. The mean +/- sem survival time of four of the horses (from discharge to the development of complications which required euthanasia) was 192 +/- 59 days (range 120 to 368 days). Routine biochemical measurements were within their normal ranges except for a high alkaline phosphatase activity (768 +/- 487 iu/litre) and a high plasma glucose concentration (7.8 +/- 0.9 mmol/litre) in 16 of the horses; only six had a glucose concentration above 9.0 mmol/litre. The basal plasma concentration of adrenocorticotrophic hormone (ACTH) in the diseased horses was at least 191 pg/ml. Eight of the horses had mean plasma cortisol concentrations before and after the administration of 25 iu of synthetic ACTH1-24 of 168 +/- 21 (range 74 to 248) and 478 +/- 58 nmol/litre (range 248 to 771), respectively. Analysis of the urine of five of the horses revealed a mean specific gravity of 1.034 +/- 0.0034 (range 1.025 to 1.045).(ABSTRACT TRUNCATED AT 250 WORDS)