We report 5 patients with AIDS who had an unusual spindle cell proliferation in the lymph nodes and skin caused by nontuberculous mycobacteriosis. The spindle cell proliferation in these tissues may mimic a spindle cell neoplasm and pose a diagnostic problem if an infectious aetiology is not suspected. The fibroblast-like spindle cells contained numerous acid fast bacilli. They were strongly positive for antibody markers of monocyte/macrophage and leukocyte derivation: Leu M3, Mo-9, T-200, and HLA-DR, and variably positive for alpha-1 anti-chymotrypsin and lysozyme. Ultrastructurally these spindle cells were predominantly fibroblast-like with poorly developed features of macrophages. These results reveal the dual macrophage and fibroblastic character of the spindle cells and probably imply a functional differentiation rather than a histogenetic one.