The clinical significance of Bence Jones (BJ) proteinuria at low concentration (less than 0.2 g/24 hours) was investigated in 33 unselected patients who had no intact monoclonal immunoglobulin in their serum. The great majority (79%) of the patients were recognized as having malignant lymphoproliferative diseases, such as chronic lymphocytic leukemia (46%), hairy cell leukemia (6%), and non-Hodgkin's lymphoma (27%), whereas only two patients (6%) had multiple myeloma or systemic amyloidosis. Five patients (15%) had no evidence of definite malignant immunoproliferative disorders at the time of recognition of BJ proteinuria. Three of them, who were excreting steadily low amounts of BJ protein in their urine for 47, 61, and 73 months, respectively, without development of any B-lymphocytic malignancy, were classified as having a monoclonal gammopathy of undetermined significance. In the remaining two patients, BJ protein disappeared spontaneously 14 and 18 months, respectively, after its recognition. The study indicates that pure BJ proteinuria, even when occurring at low concentration, is most consistently associated with malignant proliferations of B-lymphocytic origin. However, the possibility should be considered that the clinical spectrum of the monoclonal gammopathy of the light chain type also includes benign and transient forms.