In primary hyperparathyroidism, calcium homeostasis is disrupted by excessive synthesis and secretion of parathyroid hormone (PTH), which is usually caused by a solitary adenoma, or less often by nodular hyperplasia or carcinoma of the parathyroid glands. So far, the distinction between these forms of primary hyperparathyroidism has been made by histological examination. In this report clinical and histological findings, including PTH immunohistochemistry, are described in five dogs with primary hyperparathyroidism, three dogs with secondary hyperparathyroidism due to chronic renal failure, and eight control dogs. In the dogs with primary hyperparathyroidism, nodular adenomatous hyperplasia was found in two animals and parathyroid adenoma in three. The dogs with chronic renal failure had diffuse parathyroid gland hyperplasia. The parathyroid glands of the control dogs and the inactive cells surrounding the hyperplastic nodules showed slight to moderate, localized, paranuclear PTH immunolabelling. In the primary nodular and secondary diffuse hyperplasia, all parathyroid cells had a diffuse cytoplasmic PTH labelling pattern, sometimes in combination with localized paranuclear labelling. In parathyroid adenoma, areas with either paranuclear labelling or diffuse cytoplasmic labelling were observed. As both parathyroid adenoma and primary nodular parathyroid gland hyperplasia have characteristics of intrinsic autonomy (i.e., suppression of the remaining endocrine tissue), there would seem to be no functional difference between the two abnormalities. It is argued that primary (multi)nodular hyperplasia is a multiple form of parathyroid adenoma.