A study of 885 women who had given birth in Oslo, Norway in 1985 disclosed that 80% were still breastfeeding three months after delivery. Forty percent of the mothers smoked cigarettes daily. Only 7% had stopped smoking because of pregnancy or lactation. High social class and educational level was positively correlated to prolonged breastfeeding. Smoking was negatively correlated to breastfeeding when the infant was three and four months old. While 93% of non-smokers with an education of 17 or more years still breastfed four months after delivery, this was true of only 6% of heavy smokers with an education of nine years or less. Also within each social group and educational level the prevalence of breastfeeding was lower among the smokers, with a dose/response effect of the number of cigarettes smoked. More smokers than non-smokers stopped breastfeeding because they had "too little milk". Forty percent of infants breastfed by smokers suffered from colic or excessive crying, as compared with 26% of the children of non-smokers.