Between 1980 and 2000 total fertility in Kenya fell by about 40 per cent, from some eight births per woman to around five. During the same period, fertility in Uganda declined by less than 10 per cent. An analysis of the proximate determinants shows that the difference was due primarily to greater contraceptive use in Kenya, though in Uganda there was also a reduction in pathological sterility. The Demographic and Health Surveys show that women in Kenya wanted fewer children than those in Uganda, but that in Uganda there was also a greater unmet need for contraception. We suggest that these differences may be attributed, in part at least, first, to the divergent paths of economic development followed by the two countries after Independence; and, second, to the Kenya Government's active promotion of family planning through the health services, which the Uganda Government did not promote until 1995.