In a study on the differentiation between pituitary-dependent hyperadrenocorticism (PDH) and hyperadrenocorticism due to adrenocortical tumour (AT), two questions were addressed: 1. Do basal urinary corticoid/creatinine (c/c) ratios have any value in this respect, and 2. what is the reference percentage suppression of the urinary c/c ratios in the high-dose dexamethasone suppression test? Data obtained from 160 dogs with hyperadrenocorticism were analysed. In 49 dogs the diagnosis AT was confirmed by the finding of plasma ACTH concentrations < 40 ng/l, by visualisation of the tumour by ultrasonography and/or computed tomography, and by histological examination of the adrenal tissue obtained at surgery or autopsy. Among the 111 dogs with PDH, there were 31 animals with resistance to dexamethasone suppression, i.e., suppression < 50%. The basal urinary c/c ratios of dogs with PDH and AT did not differ significantly, although urinary c/c ratios > 100 x 10(-6) almost exclusively occurred in association with PDH. Among the dogs with hyperadrenocorticism, the positive predictive value of a basal urinary c/c ratio > 100 x 10(-6) for the diagnosis of PDH was 0.90 (95% CI: 0.74-0.98). Of the 49 dogs with AT, 34 had a urinary c/c ratio after dexamethasone administration higher than the basal urinary c/c ratio. The maximum suppression of the basal urinary c/c ratio in dogs with AT was 43.7%. It is concluded that in dogs with hyperadrenocorticism basal urinary c/c ratios only have predictive value in the differentiation between AT and PDH when the ratio exceeds 100 x 10(-6). The generally accepted criterion of 50% suppression by dexamethasone in the differentiation between PDH and AT is also applicable to the urinary c/c ratio.