Background. This cross-sectional study was done to assess undernutrition among children under the age of 5 years in a slum of Kolkata (n = 188) and to compare these results with z score and the conventional Indian Academy of Pediatrics (IAP) system. Methods. Data were collected by interviewing the mothers of the children, conducting anthropometric measurements, and using World Health Organization (WHO) growth charts. Statistical analysis was done using SPSS 16. The chi-square test was used, and P < .05 was considered statistically significant. Results. Overall, around 60% children were undernourished (z score < −2 SD). The conventional growth monitoring detected only underweight children, but when the z score system of WHO Child Growth Standards was used, those children were further classified as wasting and stunting. As per the IAP criteria, 104 children were undernourished and 7 were severely undernourished, but by z score, 98 children were undernourished and 29 were severely undernourished. Conclusion. The z score system identified a much higher prevalence of severe undernutrition compared with the IAP system (15.4% vs 3.7%). The conventional use of underweight (low weight for age) instead of z score as the sole criterion for identifying undernourished children may underestimate the true incidence of undernutrition in a community, which might prevent those children from receiving the deserved benefits of nutritional supplementation from different programs. More widespread use of the z score system, especially in community-based studies, may be thus recommended.