Reticulohistiocytoma (RH) is a rare benign lesion of the soft tissue. It belongs to a group of disorders called non-Langerhans cell histiocytosis and is a type of reticulohistiocytosis, all of which are types of histiocytosis. Histiocytosis is a condition in which there is rapid production (proliferation) of histiocytes (immune cells) in the skin or soft tissues. The stimulus that causes the immune system to react in RH is currently not well understood. RH present as a yellow to reddish-brown smooth surfaced, firm nodule or lesion on the trunk and/or extremities of the body. Historically, RH has been found in young adults, with a slightly higher incidence in males. RH typically resolve spontaneously over a period of months to years, are not associated with systemic disease, and do not otherwise affect health. Treatment involves surgical removal of the lesion.

While it is known that reticulohistiocytoma (RH) develop due to a rapid production of immune cells (histiocytes) in the skin or soft tissues, the cause of this process is not currently known.

There are several differential diagnoses for RH. It is important to distinguish RH from Rosai-Dorfman disease, juvenile xanthogranuloma, a variety of granulomatous conditions, and some malignant neoplasms, including histiocytic sarcoma, melanoma, and epithelioid sarcoma.

Reticulohistiocytoma should also be distinguished from multicentric reticulohistiocytosis.