In light of the variable clinical expression and bone marrow plasmacytosis of multiple myeloma, we studied prospectively the prevalence and clinical correlates of bone marrow plasmacytosis in 133 anemic medical inpatients. Seventeen patients (13%) had 5% or more, and four patients (3%) had 10% or more marrow plasma cells. Only two patients (with plasmacytosis of 7.4% and 46.2%, respectively) had multiple myeloma. Lung infections (pneumonia, tuberculosis, abscess) accounted for 41% of the cases of plasmacytosis. We found that plasmacytosis correlated with the severity of pneumonia and that active pulmonary tuberculosis was usually associated with notable plasmac. ytosis. Cancer and liver disease were not important causes of plasmacytosis in this study. There was a correlation between plasmacytosis and serum hyperglobulinemia. A correlation between advancing age of the patients and increasing prevalence of plasmacytosis was attributed in part to an increasing prevalence of serious illness with age. However, some elderly patients with apparently benign disorders had unexplained plasmacytosis.