The present study was based on data from a longitudinal research program. The cohort consisted of 12,079 children, born in the Stockholm area in 1953. There were 494 children born with low birth weight (LBW; 2500 g or less). The results of the present study showed, that the LBW children had significantly lower school marks and intelligence-test scores (numerical, verbal and logical abilities) at the age of 13 than the normal birth weight children (NBW). For girls reared in non-manual socio-economic status (SES), decreased school marks and IQ-test scores were related to birth weight, and this was especially pronounced for LBW girls born after pregnancy week 37. For boys, however, no decreased school marks and IQ-test scores were related to birth weight and gestational age, with the exception of verbal ability for LBW boys born after pregnancy week 37 reared in non-manual SES.