Summary We report two patients who developed benign plasmacytosis with multiple skin lesions. The cases were characterized by hyperplasia of mature plasma cells, and polyclonal hypergammaglobulinaemia. One patient had hyperplasia of mature plasma cells not only in the skin, but also extensively in lymph nodes and the retroperitoneal areas around the ureters. The other had plasma cell hyperplasia limited to the skin. Extensive investigations failed to reveal any clinical or laboratory evidence suggesting the presence of any underlying disease accompanying the hypergammaglobulinaemia and/or plasma cell proliferation, such as chronic infectious disease, collagen disease or other chronic inflammatory disorder. Clinically and histologically, the first patient showed features compatible with a diagnosis of systemic plasmacytosis and the second with a diagnosis of cutaneous plasmacytosis. Significant serum interleukin-6 (IL-6) levels were detected in both patients, suggesting that IL-6 may be involved in the pathogenesis of these conditions.