Patients (124) with hematological and solid neoplasms had pretreatment urinary polyamine deteminations. Putrescine, spermidine and spermine were significantly increased as compared to normals (P < 0.001). Polyamine levels were directly related to disease activity and tumor burden. In patients with multiple myeloma, putrescine levels were significantly correlated with clinical disease activity as well as the in vitro labeling index of marrow plasma cells. Spermidine values reflected tumor cell burden. Serial studies in 56 patients indicated that a greater than 2-fold rise in urinary spermidine during treatment was correlated with cell kill and subsequent clinical response (P < 0.001). Serum polyamine levels in 17 patients were comparable to urinary values. Polyamine determinations can be clinically useful, i.e., baseline values as indicators of tumor cell mass and growth fraction, and increases in spermidine during treatment as an excellent marker of tumor cell kill.