Background. Extended breastfeeding is known to benefit the health of children in developing countries and despite widespread expectations of a decline in breastfeeding in these countries, it has been demonstrated that the incidence and duration of breastfeeding are in fact increasing in many countries. Methods. In this paper, trends in breastfeeding duration are examined in 15 developing countries, using data from two comparable surveys for each country, the World Fertility Survey (conducted in the late 1970s) and the Demographic and Health Survey (conducted in the late 1980s). Multivariate regression models are used to examine differentials in breastfeeding behaviour across population subgroups in these countries for each time period, and these differentials are used to determine the extent to which the observed trends are due to changes in population characteristics and to what extent behaviour has changed within population subgroups. Results. Results show that changes in the characteristics of the population have almost universally pushed breastfeeding durations in a downward direction. On the other hand, trends within population subgroups have been positive in all but two of the 15 countries examined. Conclusions. Changes in population characteristics can be expected to continue for most developing countries, exerting a downward pressure on breastfeeding. Policies that promote breastfeeding are needed to counter these changes, especially in the most vulnerable population subgroups.