To investigate whether lithium ameliorates the infectious complications that accompany systemic chemotherapy, we studied 45 patients with small-cell bronchogenic carcinoma receiving combination chemotherapy and radiation therapy. Twenty received lithium carbonate, and 25 received no additional therapy. Control subjects experienced more days with neutropenia than the lithium-treated group (2.17 days per 100 patient-days vs. 0.29), more severe febrile episodes (seven patients vs. one patient), more days hospitalized with fever and neutropenia (1.92 per 100 patient-days vs. 0.18), and more infection-related deaths (five vs. none). Infection-free survival was significantly longer in the lithium-treated group than in controls (P less than 0.05). Delay in subsequent chemotherapy was longer (P less than 0.01) and the number of dose reductions greater (P less than 0.01) in the control group. For both leukocytes and neutrophils, the first cycle nadir, mean of all treatment nadirs, and the lowest nadir observed during treatment were significantly higher in the lithium group. Mean mid-cycle monocyte counts were greater in the lithium group (P less than 0.05) and correlated with concurrent serum lithium levels (rs = 0.74, P less than 0.05). We believe that lithium carbonate shows promise as a means of lowering the risk of infection among patients receiving cytotoxic therapy.