Serial data from 504 infants were used to develop reference data for 1-mo increments in weight and recumbent length by fitting a family of three-parameter mathematical functions to the serial data for each individual. The percentiles for status and for increments tended to be larger for the boys than for the girls except for the lower percentiles of increments for weight after 9-10 mo and for recumbent length after 8-9 mo. These data can be used to assess growth in white or black infants whether they are breast-fed or formula-fed. These new data will allow earlier evaluation of growth velocity than the reference data for 3- and 6-mo increments that were available previously. This can lead to the earlier initiation of diagnostic procedures and treatment. The present data should be useful also in research studies.