The prevalence of variables frequently associated with low birthweight was compared in samples of low-birthweight (LBW) infants and normal-birthweight infants (single births of the white race) drawn from the Boston [Massachusetts, USA], component of the Collaborative Perinatal Research Project. Then the association between these exposure variables and CNS function at 7 yr was examined in the LBW infants. Finally the interrelationships of the 5 variables having the strongest association with abnormal or suspect CNS function were examined with regard to their joint predictive utility. A high-risk subgroup (10%) of the total LBW group having a greater than 50:50 chance of falling into an adverse category at age 7 was identified by various combinations of the factors: gestational age of 33 wk or less, premature rupture of membranes, low Apgar score and respiratory distress. Small subgroups thus defined should receive special attention in follow-up, particularly in relation to the new regional perinatal care centers. They might also be selected for special study of the mechanisms of interference with CNS development. In a population of 7-yr-old children with abnormal or suspect CNS function as defined by the protocol of this study, 80% will have a history of normal birthweight.