Objective Child undernutrition is a serious public health problem in many low- and middle-income countries. Data on child undernutrition prevalence and its risk factors in Armenia are limited. The present study aimed to estimate the prevalence and explore the predictors of undernutrition among children aged 5-17 months in Yerevan. Design The study was cross-sectional and employed a review of the ambulatory charts of children selected through a multistage cluster sampling. This phase was followed by a case-control study. The cases were undernourished children identified during the record review and randomly matched with normally growing controls of the same age and gender from the same pool of records. Mothers of cases and controls participated in a telephone interview. The study used conditional logistic regression analysis. Setting Yerevan, Armenia. Subjects Children aged 5-17 months residing in Yerevan, Armenia. Results Review of 570 ambulatory charts suggested the prevalence of stunting, underweight and wasting among 5-17-month-old children in Yerevan to be 17 center dot 9 %, 7 center dot 3 % and 3 center dot 1 %, respectively. The case-control study of eighty-nine matched pairs identified four significant predictors of child undernutrition: family's socio-economic status score (P = 0 center dot 030), child's length at birth (P = 0 center dot 027), duration of predominant breast-feeding (P = 0 center dot 046) and food diversity score (P = 0 center dot 039). Conclusions The factors determining growth patterns of children in Yerevan are mostly behavioral and environmental, hence modifiable. Reducing poverty and inequalities in food availability, promoting breast-feeding and adequate complementary feeding, and ensuring optimal care before, during and after pregnancy are likely to help reduce child undernutrition in Yerevan, Armenia and societies with similar public health concerns.