Mortality during the first two years of life among 1476 Pakistani infants born between September 1984 and March 1987 is described according to age, causes of death, area of living, season of the year and sex. The mortality rate (deaths under two years/1000 live births) projected over the whole population of Pakistan was 127 and under one year 114. During the first two years of life, the mortality rate was 133/1000 in the village, 159 in the periurban slum, 107 in the urban slum and 17 in the upper middle class group. In the latter group all deaths had occurred within 72 hours after birth. The overall major causes of death were acute and prolonged diarrhoea (36%), asphyxia neonatorurn (13%), respiratory infections (13%), septicaemia (11%) and tetanus (9%). A clear age dependency was noted with 14% of deaths occurring during the first 24 hours of life (asphyxia neonatorum in 86%), and 57% dying within the first 28 days of life. In the later age groups, infections were mainly responsible for 82% of total deaths. Early mortality was therefore extremely high in the poorer areas studied and the cause of death was highly age dependent. Any interventions for reducing mortality must therefore be directed towards better antenatal care and safe delivery and postnatally, towards preventing infections, especially during the first six months of life.