Summary Using data from a longitudinal study conducted in 40 villages by the UK MRC in the North Bank Division of The Gambia beginning in late 1981, we examined infant and child mortality over a 15-year period for a population of about 17 000 people. Comparisons are drawn between villages with and without PHC. The extra facilities in the PHC villages include: a paid Community Health Nurse for about every 5 villages, a Village Health Worker and a trained Traditional Birth Attendant. Maternal and child health services with a vaccination programme are accessible to residents in both PHC and non-PHC villages. The data indicate that there has been a marked improvement in infant and under-five mortality in both sets of villages. Following the establishment of the PHC system in 1983, infant mortality dropped from 134/1000 in 1982–83 to 69/1000 in 1992–94 in the PHC villages and from 155/1000 to 91/1000 in the non-PHC villages over the same period. Between 1982 and 83 and 1992–94, the death rates for children aged 1–4 fell from 42/1000 to 28/1000 in the PHC villages and from 45/1000 to 38/1000 in the non-PHC villages. Since 1994, when supervision of the PHC system has weakened, infant mortality rates in the PHC villages have risen to 89/1000 in 1994–96. The rates in the non-PHC villages fell to 78/1000 for this period. The under-five mortality rates in both sets of villages have converged to 34/1000 for 1994–96. When the PHC programme was well supported in the 1980s, we saw significantly lower mortality rates for the 1–4-year-olds. These differences disappeared when support for PHC was reduced after 1994. The differential effects on infant mortality are less clear cut.