An important part of clinical endocrinology deals with diseases caused by benign and malignant nodules growing within originally homogeneous endocrine glands. In the search for the pathogenesis of these nodules, two concepts have been advanced: either the hyperplastic tissue is considered to result from chronic intense stimulation by a trophic hormone, eventually causing the growth of polyclonal nodules (a concept known as NNEH) or else, the nodules are thought to represent true clonal tumors. It has been realized rather recently that NNEH accounts only for a small minority of rare endocrine diseases, with the exception of highly prevalent iodine deficiency goiters and, in a broader sense, for Graves' disease. Indeed, secondary hyperplasia of endocrine glands resulting from long-lasting chronic hormonal overstimulation cannot explain a large number of essential features of the diseased glands. As best documented for the thyroid gland, outstanding among the characteristics of hyperplastic glands that do not fit into the simple concept of NNEH are the inevitable nodular transformation, the frequent autonomy and irreversibility of nodular growth, the loss of function in many cells and nodules, and the tremendous regional heterogeneity of growth and function including loss of coordination between these two main features of any living cell. Hyperplasia resulting from long-lasting stimulation by a trophic hormone is not a fully reversible process, as is often thought, but definitely increases the total cell mass and leaves behind, after cessation of the stimulus, a population of newly generated cells. This process is common to NNEH and true neoplastic growth. A review of common and disparate traits between endocrine hyperplasia and neoplasia must be based on the following generally accepted facts (excluding in-depth consideration of malignancy). 1. Many endocrine tumors are clonal while others are polyclonal. For example, pituitary and sporadic parathyroid adenomas are nearly always clonal while in the thyroid, multiple clonal nodules of different origin may coexist with polyclonal nodules. 2. Clonal growth does not necessarily indicate that the autonomous expansion of a tumor is driven by presently known genetic gain-of-function or loss-of-inhibition mutations or by cytogenetic aberrations. 3. Genetic mutations and chromosomal aberration in endocrine tumors are not necessarily primary events in the tumorigenesis but may as well be a late secondary phenomenon in growing tissue. 4. Clonal nodules may overgrow from primarily polyclonal ones. The best evidence to date comes from the rare clonal parathyroid adenomas with allelic loss of chromosome 11 evolving in tertiary hyperparathyroidism.(ABSTRACT TRUNCATED AT 400 WORDS)