A cohort of 312 children followed in three rural primary care centers was studied. All cases of failure-to-thrive were identified (30) and compared to the remainder of the group. With the use of data from medical records, children with history of failure-to-thrive were found to be significantly lighter and shorter than the remainder of the population in each of the first five years of life. More cases had family problems. No differences were found in demographic characteristics or chronic illnesses. In the second phase of the study, cases and an equal-sized control group, from the same social setting, matched for age, sex, mother's marital status, and family problems, were examined at 3 to 6 years of age to assess outcome. This included physical examination, the McCarthy Scale of Children's Abilities, a behavior questionnaire, and an interview to assess life events. Cases were lighter but not shorter than controls. No instances of occult organic disease causing failure-to-thrive were identified. No behavioral or developmental deficits could be attributed to the failure-to-thrive.