AIM: To review the methods for generating windows of achievement for six gross motor developmental milestones and to compare the actual windows with commonly used motor development scales. METHODS: As part of the WHO Multicentre Growth Reference Study, longitudinal data were collected to describe the attainment of six gross motor milestones by children aged 4 to 24 mo in Ghana, India, Norway, Oman and the USA. Trained fieldworkers assessed 816 children at scheduled visits (monthly in year 1, bimonthly in year 2). Caretakers also recorded ages of achievement independently. Failure time models were used to construct windows of achievement for each milestone, bound by the 1st and 99th percentiles, without internal demarcations. RESULTS: About 90% of children achieved five of the milestones following a common sequence, and 4.3% did not exhibit hands-and-knees crawling. The six windows have age overlaps but vary in width; the narrowest is sitting without support (5.4 mo), and the widest are walking alone (9.4 mo) and standing alone (10.0 mo). The estimated 1st and 99th percentiles in months are: 3.8, 9.2 (sitting without support), 4.8, 11.4 (standing with assistance), 5.2, 13.5 (hands-and-knees crawling), 5.9, 13.7 (walking with assistance), 6.9, 16.9 (standing alone) and 8.2, 17.6 (walking alone). The 95% confidence interval widths varied among milestones between 0.2 and 0.4 mo for the 1st percentile, and 0.5 and 1.0 mo for the 99th. CONCLUSION: The windows represent normal variation in ages of milestone achievement among healthy children. They are recommended for descriptive comparisons among populations, to signal the need for appropriate screening when individual children appear to be late in achieving the milestones, and to raise awareness about the importance of overall development in child health.