Parathyroid adenoma and hyperplasia are the most common causes for hyperparathyroidism, and distinction between them is controversial based on the current criteria for pathological diagnosis. We studied the clonality of hyperparathyroidism and its correlation with the pathological features, analysing 39 female patients with hyperparathyroidism. Clonality was successfully detected in 12 heterozygous cases by PCR amplification of PGK-1 gene. The 12 cases yielded 14 hypercellular glands, 8 affected by primary and 6 by secondary hyperparathyroidism. The results revealed that 7 of the 8 glands with primary hyperparathyroidism showed monoclonal proliferation. Only 1 gland pathologically diagnosed as adenoma showed a polyclonal pattern. In the 4 cases with secondary hyperparathyroidism, at least one monoclonal tumour was detected in each case. Our data indicate that monoclonal tumours are more common than expected in both primary and secondary hyperparathyroidism. Monoclonal tumours and polyclonal hyperplasia can co-exist in the same patient. Comparative study of the clonality and the pathological features showed that the clonality was consistent with the diagnosis of parathyroid adenoma, whereas it was in conflict with the diagnosis of hyperplasia with multigland involvement. One of the reasons for this is that we are ignorant of the true natures of hyperparathyroidism with multigland involvement.