The ages at attainment of 12 gross motor milestones were obtained prospectively during well-baby visits in the first two years of life for 381 children. All had been born at term and were judged to be normal at one year. A longitudinal analysis, using an index summarizing each child's progress for eight selected milestones, is reported for the 284 children for whom data were complete. A high percentage of parents were able to report the age at attaining milestones with an acceptable degree of variability. The children attained milestones at earlier ages than traditionally reported. There were only minor sex differences in age at attainment, but black children attained milestones earlier than white children.