The term “junctional melanocytic hyperplasia” (JMH) refers to the proliferation of melanocytes along the dermo–epidermal junction that is the basal layer where normal melanocytes reside. JMH is not a clinical diagnosis but a histological term, just as epidermal hyperplasia or nodular lymphocytic infiltrate . JMH generally refers to the proliferation of single melanocytes rather than nests of melanocytes, and is characteristic of lentigo simplex; hence, the term lentiginous is often used to describe this finding.

Some pathologists make the diagnosis of lentiginous nevus for a junctional nevus with prominent lentiginous (or single cell) melanocytic hyperplasia. Others use the term jentigo to describe such proliferation, implying a lesion with features of both lentigo and early junctional nevus. Clinically, these lesions are usually very dark but small. Biologically, they represent the early stages of a melanocytic nevus and are common in childhood and youth. It is their dark and sometimes black color that causes concern by the patient, leading to their excision. Both, lentigo simplex and lentiginous junctional nevus are easily diagnosed histologically .

As mentioned in the discussion on dysplastic nevus , JMH is a key histological feature in dysplastic nevi and melanoma.

Many times, the pathology report for a biopsy specimen in which the clinician suspected solar lentigo versus lentigo maligna carries the diagnosis of “atypical junctional melanocytic hyperplasia.” This is a histological conceptual diagnosis and not a clinical diagnosis, at least not for the time being.

In order to understand this concept, it is important to go to the other end of the spectrum of junctional melanocytic proliferations, namely melanoma in situ.