ABSTRACT.The relationships between breast‐feeding practices and the rates of gastro‐intestinal and lower respiratory illness during the first two years of life were examined for a birth cohort of New Zealand infants. During the first four months, there were significant tendencies for rates of gastro‐intestinal illness to decrease with increasing duration of breast feeding. These trends remained significant when the effects of a number of social and familial factors were taken into account. There was no association between duration of breast‐feeding and rates of gastro‐intestinal illness beyond four months. Prolonged breast‐feeding was associated with significantly lower rates of lower respiratory illness during both the first and second years. However, when the effects of social and familial factors were taken into account the apparent associations between duration of breast‐feeding and rates of lower respiratory illness became non‐significant. The implications of these finding