A sample (1,031 individuals) of infants born at varying gestational ages (26–42 weeks) and birth weights (860–4,750 g) was used in a study of 5 developmental traits appearing during the first 2 years of life: the ages at which a child could turn over, sit up, stand up, walk without support, and eruption of the first tooth. The mean age of attaining each developmental milestone was not significantly different among the Jewish ethnic groups represented in the sample. In addition, the following potential predictor variables were evaluated: the infant's weight, length, and head circumference at birth and at 1.5 years of age; maternal body weight, height at the infant's birth, and number of miscarriages; family size and parental occupation. Only few variables showed significant effects with developmental milestone; birth weight made the greatest contribution. Highly significant intraclass and Pearson correlations were observed between siblings in the ages of appearance of each of the 5 developmental milestones. The correlations ranged between 0.30 and 0.53 for the total group, after adjusting for all significant covariates. © 1992 Wiley‐Liss, Inc.