This review reports the beneficial effects, observed in our clinical studies, of Bifidobacterium breve for premature infants, and children with cancers undergoing chemotherapy. To investigate the protective effects of B. breve (M-16V) as a probiotic on necrotizing enterocolitis (NEC) and infection in premature infants, we carried out a clinical study in 338 very low birth weight infants over a five-year period. These patients were supplemented with B. breve starting several hours after birth (Bifido group). 226 premature infants served as controls. Infants of the Bifido group were administered B. breve in a daily dose of 1×10(9) cells/day. The incidence of NEC was significantly reduced in the Bifido group (nil) compared with that in controls (6 cases, P<0.01). Infection also decreased significantly. Thus, administration of B. breve as a probiotic looks to be a very effective treatment for preventing NEC and infection in preterm infants. Mucositis, also referred to as mucosal barrier injury, is one of the most debilitating side effects of chemotherapy treatment. To evaluate the effects of the administration of B. breve (BBG-01, another strain than that used in the study of premature infants), a clinical study was performed to ascertain whether it attenuated intestinal mucositis in children with cancers on chemotherapy. A placebo-controlled trial was performed in patients with malignancies admitted for chemotherapy (n=42), who were randomised into two groups receiving probiotic or placebo. The frequency of fever and the use of intravenous antibiotics were significantly lower in the Bifido group than the placebo group. The B. breve administration enhanced the colonisation of anaerobes. Disruption of the intestinal microbiota after chemotherapy, such as the increase in the population levels of Enterobacteriaceae, was more pronounced in the placebo group. In conclusion, these data suggest that administration of B. breve is an effective approach to attenuating chemotherapy-induced mucositis in children with cancers. The study results strongly suggest that B. breve administration as a probiotic is an effective therapy for the prevention of NEC and infection in premature infants, and also a promising treatment for attenuating chemotherapy-induced mucositis in children with cancers.