The 1st yr of life is an age when morbidity and medical care use is high, and this is particularly true for low birth weight infants. Whether certain factors characterize subgroups at especially increased risk was examined for a large random sample (n = 4989) of 1 yr old infants by using rehospitalization as the dependent variable. Overall, 9.1% of the infants were rehospitalized and this increased with decreasing birth weight to 38.2% of those .ltoreq. 1500 g at birth. Low birth weight infants accounted for 6.4% of 1 yr olds but 13.6% of those hospitalized and 20.0% of all hospital days among these infants. Factors affecting the chances of rehospitalization for low birth weight and normal birth weight infants, with and without congenital anomalies/developmental delay, were similar. Maternal hospitalization during pregnancy, prolonged postnatal stay of the infant, variables indicative of low socioeconomic status and certain types of medical care use were associated with increased risk of hospitalization. The risk of hospitalization associated with some variables was high, but it was not possible to identify with precision a group where reduction in hospitalization would result in major decreases in overall hospital use by infants.