Owing to its high contents of fructooligosaccharides (FOSs), the yacon (Smallanthus sonchifolius) root is used in traditional Andean medicine as a substitute for cane sugar in diabetes and for obesity prevention. This study was designed to test the hypothesis that regular consumption of yacon works to improve the immune system. BALB/c mice were fed with the AIN-93 diet supplemented with 5% commercial FOS or either 3% or 5% yacon FOS for 30 consecutive days. Animals in the control group were fed with nonsupplemented ration. Food intake; weight gain; serum levels of IgA, IgM, and IgG; levels of fecal IgA, production of nitric oxide by peritoneal macrophages, frequencies of T and B lymphocytes in the spleen and peripheral blood, T-cell proliferation, and cytokine production were evaluated in all groups. No significant differences were observed in food intake and weight gain when the experimental and control groups were compared. Also, serum levels of IgA, IgM, and IgG; nitric oxide production in peritoneal macrophages; frequencies of T and B lymphocytes in the spleen and peripheral blood; T-cell proliferation; and production of interleukin (IL)-4, interferon-γ, IL-10, and tumor necrosis factor α did not differ in the different groups. The intake of FOS, however, led to a significant reduction of the proinflammatory cytokine IL-1. β in macrophage cultures and elevation of the levels of fecal IgA. Together, these results indicate that the daily consumption of yacon does not exert negative effects on the immune system, helps to preserve an anti-inflammatory state in phagocytic cells, and improves mucosal immunity, possibly preventing the risks associated with autoimmune and metabolic diseases. © 2012 Elsevier Inc..