Preclinical studies have established that the polysaccharide fractions of Ganoderma lucidum have potential antitumor activity. Recent clinical studies have demonstrated that G. lucidum polysaccharides enhance host immune functions [e.g., enhanced natural killer (NK) cell activity] in patients with advanced solid tumors, although an objective response was not observed. This open-label study aimed to evaluate the effects of water-soluble G. lucidum polysaccharides (Ganopoly, Encore International Corp., Auckland, New Zealand) on immune functions in patients with advanced lung cancer. Thirty-six patients were enrolled and treated with 5.4 g/day Ganopoly for 12 weeks. In the 30 cancer patients who completed the trial, treatment with Ganopoly did not significantly alter the mean mitogenic reactivity to phytohemagglutinin, mean counts of CD3, CD4, CD8, and CD56, mean plasma concentrations of interleukin (IL)-2, IL-6, and interferon (IFN)-gamma, or NK activity in the patients, but the results were significantly variable. However, some cancer patients demonstrated markedly modulated immune functions. The changes in IL-1 were correlated with those for IL-6, IFN-gamma, CD3, CD8, and NK activity (P < .05), and IL-2 changes were correlated with those for IL-6, CD8, and NK activity. The results suggest that subgroups of cancer patients might be responsive to Ganopoly in combination with chemotherapy/radiotherapy. Further studies are needed to explore the efficacy and safety of Ganopoly used alone or in combination with chemotherapy/radiotherapy in lung cancer patients.