The Functional Independence Measure for Children (WeeFIM™) instrument was developed, based on the FIM™ instrument, to assess disability in children aged 6 mo to 7 yr. Normative data are reported for American and Japanese children, and it is increasingly used for the disabled. Our purpose was to confirm scale quality and to determine the difficulty pattern of the WeeFIM in Japan. The WeeFIM was measured in 225 children (113 girls and 112 boys) aged 6 mo to 7 yr without developmental delays. The scores were converted to an interval scale by Rasch analysis, which assumes unidimensionality of the measurement items, determines the degree of the fit to the assumption, and decides item difficulty. When the WeeFIM items were divided into two groups of motor and cognitive items to minimize misfit, the degree of misfit was acceptable, except for eating, bladder management, tub/shower transfer, and comprehension. For the motor items, grooming, bathing, and bladder control were more difficult, and stairs, bed/chair transfer, and walk or wheelchair were easier. Concerning the cognitive items, expression and comprehension were easier, and problem-solving was most difficult. When we compared item difficulty patterns in the four age groups (6-21 mo, 22-45 mo, 46-62 mo, and 63-100 mo), we found no differences, except in toilet transfer. It was more difficult for younger children, possibly because of its different pattern of chronological change, which shows rapid changes from dependent to independent levels over a short period of time. Our study confirmed the scale quality of the WeeFIM instrument with Rasch analysis and demonstrated the difficulty pattern of the WeeFIM in nondisabled Japanese children.