Breast milk is the optimum for all infants, but hospitalization in the neonatal intensive care unit can cause separation of mothers and infants, which often interferes with milk secretion. Some reports show that domperidone is effective in promoting milk secretion. However, the Food and Drug Administration in the United States cautioned to not use domperidone for increasing milk volume because domperidone carries some risk of cardiac events, including QT prolongation, cardiac arrest, and sudden death. In contrast, it is used in Canada, Australia, and the United Kingdom with safety. The pharmacodynamics and pharmacokinetics of drugs may vary by race or ethnic origin, and it is not known whether domperidone is effective or safe for Japanese. In this study we report the effects of domperidone for Japanese mothers with insufficient lactation. Ten mothers were enrolled in a pilot study. After confirming that there were no abnormal findings on the electrocardiogram, the mothers were administered domperidone. Seven of 10 who took domperidone increased their milking volume. Prolactin was increased in 9 of 10 mothers. Adverse events were observed in two mothers, one headache and one abdominal pain; all symptoms were mild and improved promptly; and there were no adverse cardiac events. These results are consistent with reports from other countries. Domperidone may tentatively be considered effective for increasing milk secretion in Japanese mothers as in other populations. Our preliminary study of 10 cases indicates the need for further studies with larger sample sizes to assess the efficacy and safety of domperidone.