Longitudinal studies of morbidity from infectious diseases and physical growth were completed from July 1982-June 1984 for 153 Peruvian newborns during the first year of life. Admission to the cohort was restricted to infants with birth weights greater than 2, 500 g. Surveillance workers inquired about symptoms of diarrheal, respiratory, and other illnesses during thrice-weekly home visits; an thropometrists measured weight and length each month to assess the infants' patterns of physical growth and relative nutritional status. During 48, 209 childdays of observation, upper respiratory infections were present on 13, 409 childdays (27.8% prevalence) and diarrhea on 7, 466 child-days (15.5% prevalence). The diarrhea incidence rate averaged 9.8 epIsodes per child-year of observation; all children had at least one episode of diarrhea. Average weights approximated those of the National Center for Heatth Statistics reference population during the first five to six months, but declined thereafter in relation to reference data. Average lengths were less than the reference data at all ages. The average weights for age and lengths for age of gills were generally greater than those of boys relative to the reference population. Rates of stunting and wasting Increased progressively during the first year of life. © 1989 by The Johns Hopkins University School of Hygiene and Public Health.