Mothers breast feeding 2 weeks after delivery were studied. One group (106 mothers) were advised to 'eat for two' during lactation to sustain their milk supply. They were compared at 3 months with a control group of 152 mothers. At 3 months only half as many advised group mothers had weaned their babies due to insufficient milk as control mothers. The numbers weaning for other reasons were similar in both groups. More breast feeding mothers reported an increased appetite during lactation than did mothers who had changed to artificial feeding. Smokers were more likely to have given up breast feeding between 2 weeks and 3 months than non-smokers but smoking was not associated with any particular weaning reason. Women taking a contraceptive pill during lactation more often experienced a diminution of milk supply than non-pill-takers. Sustained lactation benefits the mother since those who were still breast feeding were less likely to suffer from depression or fatigue at 3 months, and were more likely to have lost weight. The early introduction of solids was less frequent among infants being breast fed at 3 months.