We analyzed the characteristics of weight growth and present the weight growth charts from birth to 6 years of age in Japanese triplets. The study included 366 mothers and their 1098 triplet children, who were born between 1978 and 2006. Data were collected through a mailed questionnaire sent to the mothers asking for information recorded in medical records. For these births, data on triplets' weight growth, gestational age, sex, parity, and maternal age at delivery were obtained from records in the Maternal and Child Health Handbooks, which is provided by the authorities after a report of pregnancy. Birthweight proved to be the strongest contribution on weight of triplets from 1 to 6 years of age. In addition, gestational age was also a significant contributing factor to weight from birth to 6 years of age. Moreover, males had a higher weight from birth to 6 years of age than females. Compared to the 50th percentile of the growth standard for the general population of Japan, the weight deficit of the triplets was more than 40% at birth (male, –1.28 kg; female, –1.28 kg), decreased within the first 1 year of age, and fluctuated between 4% and 9% until 6 years of age (male, –1.82 kg; female, –1.78 kg). In conclusion, triplets have lower birth weight than singletons and in spite of the rapid catch-up growth during first year of life they are behind singletons even in mid-childhood. This study provides growth curves for use in triplets.