Histological study of 175 prostates with or without nodular hyperplasia was carried out. As to the commonest site of the involvement of prostatic hyperplasia, the inner part of the lateral lobe as well as the middle lobe has been described by numerous investigators. The former has been usually called as the inner gland, but the present author failed to confirm any histological difference of the epithelium or a boundary between the inner and the outer gland.The earliest demonstrable lesion of nodular hyperplasia was composed of stromal and epithelial hyperplasia, being classified into five histological types.Type Ia was a localized lesion purely composed of fibroblasts associated with diffuse lymphocytic infiltration.Type Ib had the same histological features as type Ia, except for a finding with some proliferation of smooth muscle in the lesion of fibroblastic hyperplasia.These two types of stromal hyperplasia were induced in the periurethral layer of the inner gland and frequently observed in the aged group after 60 years old.Type IIa was represented by epithelial hyperplasia of the distal duct without any stromal proliferation. The duct had a crescent-like cavity with a characteristic epithelial growth resulting in a papillary formation outwards from the inner side of the crescent. The lesion was usually induced in the middle layer of the inner gland and in the middle lobe.Type IIb consisted of lobular hyperplasia without signs of stromal proliferation. The lesion appeared in the 5th decade and increased its incidence with aging. Lobular hyperplasia was usually observed in the outer layer of the inner gland and also in the middle lobe.Glandular hyperplasia was further divided into two types, ductal and lobular, which was first described by the present author according to the usual manner of similar lesions in other organs. It was highly suggested that ductal and lobular hyperplasia developed to a typical adenomatous nodule, or to a fibroadenomatous one when the stroma was involved in hyperplasia.Type III showed combined stromal and epithelial hyperplasia, which was composed of ductal or lobular hyperplasia associated with proliferation of fibrous or fibromuscular stroma. The commonest site involvement is the middle layer of the inner gland as well as the middle lobe.In conclusion, hyperplasia of the prostate would be induced pluralistically from the stroma and the epithelium, most commonly in the so-called inner gland and in the middle lobe with the process of aging.