The study analyses various sex differences observed among children in WFS and DHS surveys conducted in sub-Saharan Africa. Four outcome indicators are presented: neonatal mortality, postneonatal mortality, child mortality and prevalence of malnutrition. Three indicators of health seeking behaviour are presented: vaccination, oral rehydration therapy and duration of breast-feeding. The statistical analysis focuses on comparison of the observed distributions of the sex ratios to theoretical distributions expected from the sample sizes of the various surveys. The results show the absence of any difference in health seeking behaviour between boys and girls, whether for preventive medicine (vaccination), curative medicine (oral rehydration therapy) or feeding practices (breast-feeding). On the other hand, mortality appears to be consistently higher for boys, more so among neonates (+28%) than among 1-12-month-olds (+8%) and 1-4-year-olds (+4%), and similarly malnutrition appears to be more prevalent among boys (+5%%). The contrast between the lack of differences in behavioural indicators and the significant differences in outcome indicators suggests a biological causality. Results are discussed in the light of biological factors.