Objective: Infants aged 0–5 months are not systematically included in assessments of child nutritional status and are generally excluded from surveys conducted in emergencies. We estimated the impact of excluding 0–5-month-old infants on the prevalence of stunting, wasting and underweight among children under 5 years (U5) and under 3 years (U3) of age. Design: Comparison of the prevalence of stunting, wasting and underweight in U5 and U3 with or without inclusion of the age group 0–5 months. Setting: Demographic and Health Surveys and Multiple Indicator Cluster Surveys from 76 developing countries and countries in transition. Subjects: Children under 3 or under 5 years of age included in the surveys. Results: Excluding 0–5-month-old infants resulted in an overestimation of the prevalence of stunting, wasting and underweight in U5 of 3.0, 0.3 and 2.6 percentage points, respectively, and of 4.8, 1.0 and 5.2 percentage points, respectively, in U3. The overestimation for wasting was negligible. The regions showing the highest overestimations for stunting and underweight were Asia and sub-Saharan Africa. Overall, countries with high prevalence of stunting and underweight showed especially large overestimations. The prevalence of underweight in infants aged 0–5 months was correlated with the prevalence of low maternal body mass index. Conclusion: All surveys, even in situations of nutrition emergency, should include 0– 5-month-old infants. Strictly comparable age ranges are essential in nutrition surveys for monitoring trends and evaluating programme impact. Greater awareness of prenatal and early child undernutrition is needed among policy-makers.