Assessments of infant sociability, motoric capacities, and nutritional factors were performed to elucidate early influences on infant development in an undernourished Kenya population. In this longitudinal study, infant social skills were assessed using both the Bayley Behavior Record and home observations during the first 6 months of the infants' lives. Abilities were measured using an adapted version of the Bayley scales of infant development at 6 and 30 months, and a short battery of cognitive measures at 5 years. Infant size measurements were used as indicators of infant nutritional status. Shorter and lighter infants, and those infants showing growth faltering in weight attainment, were significantly less sociable at 6 months than infants who were taller and maintained heavier weights. Infants with smaller arm circumferences displayed lower motor scores at 6 months and lower Bayley mental scores at 30 months than infants with greater arm circumference. Furthermore, infants who were more motorically able and socially adept at 6 months had higher Bayley scores at 30 months and higher verbal comprehension scores at 5 years.