The reasons of the increase in the LBW rate in Poland from 8.1% in 1985 to 8.4% in 1990 was investigated using the individual records from the birth certificates. There was differential increase of the low birth weight among the social groups. The highest increase was observed among the less educated mothers, especially in big cities. The LBW among the newborns delivered by mothers who had finished their education at the primary school level changed from 10.6% (in big cities from 14.7%) in 1985 to 12.5% (in big cities to 16.2%) in 1990.