Undernutrition continues to be a primary cause of ill-health and premature mortality among children in developing countries. This paper examines how the prevalence of undernutrition in children is measured and argues that the standard indices of stunting, wasting and underweight may each be underestimating the scale of the problem. This has important implications for policy-makers, planners and organizations seeking to meet international development targets. Using anthropometric data on 24 396 children in India, we constructed an alternative composite index of anthropometric failure (CIAF) and compared it with conventional indices. The CIAF examines the relationship between distinct subgroups of anthropometric failure, poverty and morbidity, showing that children with multiple anthropometric failures are at a greater risk of morbidity and are more likely to come from poorer households. While recognizing that stunting, wasting and underweight reflect distinct biological processes of clear importance, the CIAF is the only measure that provides a single, aggregated figure of the number of undernourished children in a population.