The attainment of gross motor milestones is an important indicator of motor development in early life; however, little is known about factors affecting gross motor development in children from developing countries. The purpose of this study was to examine the relation of nutritional factors (physical growth and dietary intake) and morbidity during the first year of life to the age of walking without support. Multivariate regression models were used to analyze data collected prospectively between 1991 and 1999 in rural Guatemala. Attainment of children's gross motor milestones was assessed monthly by trained field workers using the 17-milestone Gross Motor Development Scale, morbidity was assessed by biweekly recall, and dietary intakes were measured at 9 and 12 mo of age using repeated 24-h dietary recalls. Median age of walking was 15 mo (range 10-24 mo; n = 174) with no differences by gender. Models were adjusted for birth order, gender, gestational age, maternal age and education, socioeconomic status, and community. Growth in length (-0.57 +/- 0.27 mo length for age Z-score; P = 0.04) and weight (-0.54 +/- 0.19 mo weight for age Z-score, P = 0.005) during the first year of life, rather than size at birth, predicted age of walking. Animal protein intake from complementary foods, while low (mean < 1 g/d) overall, was positively associated with earlier age of walking (P = 0.02). Morbidity during infancy was not associated with age of walking. These findings indicate the importance of prevention of postnatal growth retardation and improvement of diet quality for children's gross motor development.