OBJECTIVE: To compare mothers' perceptions of their own infants' nutritional status with anthropometric indicators of undernutrition. DESIGN: A qualitative study and cross-sectional quantitative survey. The qualitative study involved developing tools to assess mother's perception. Two methods of verbal description and a pictorial scale were developed. The quantitative survey involved measuring maternal perception and comparing it with the anthropometric measures of weight-for-age Z-score (WAZ) and mid-upper arm circumference-for-age Z-score (MUACZ). SETTING: A rural community setting in Kenya. SUBJECTS: Seventy-four infants aged between 4 and 6 months, and their mothers, living in rural Kenya were enrolled. RESULTS: Using verbal description, the positive and negative likelihood ratios were 3.57 (95 % CI 1.44, 9.98) and 0.69 (95 % CI 0.50, 0.96) respectively for MUACZ<-2; and 4.60 (95 % CI 1.60, 13.3) and 0.67 (95 % CI 0.49, 0.92) respectively for WAZ<-2. Using the pictorial scale, the positive and negative likelihood ratios were 8.30 (95 % CI 1.91, 36.3) and 0.69 (95 % CI 0.52, 0.93) respectively for MUACZ<-2; and 4.31 (95 % CI 1.22, 15.0) and 0.78 (95 % CI 0.61, 1.00) respectively for WAZ<-2. CONCLUSIONS: In a rural community, mothers better identify undernutrition in their infants using a pictorial scale than verbal description. However, neither can replace formal anthropometric assessment. Objective anthropometric tools should be validated for identification of severe acute malnutrition among infants aged less than 6 months.