The large number of child deaths in developing countries is associated with delays in care-seeking by families, but the community-based efforts of the Integrated Management of Childhood Illnesses developed by WHO/UNICEF has remained ineffectual. To improve caregivers' ability to recognise potentially life-threatening symptoms of major childhood illnesses, we provided education about the importance of danger signs and immediate care-seeking practices through a community-based intervention of 'the Growth Monitoring Programme Plus (GMP+)' in low-income areas of Lusaka, Zambia. Using repeated cross-sectional data from interviews, we compared attendance and non-attendance groups to assess the impacts of intervention on mothers' care-seeking. Of 1717 and 1546 attendance mothers in the baseline and the final survey, 1097 and 1035, respectively, sought care from a health centre after perceiving the danger signs. The proportion of mothers with immediate response increased from 35.7% (392/1097) to 51.5% (533/1035) (p < 0.01). In the final survey, the attendance mothers became more likely to respond immediately to the danger signs than the non-attendance mothers (adjusted odds ratio: 2.140, 95% confidence interval: [1.408-3.252]), and the higher educational level the attendance mothers had, the more likely they were to respond immediately to the danger signs (primary level: 2.067 [1.050-4.068], secondary level and above: 2.174 [1.098-4.306]). In conclusion, GMP+ with danger sign education can improve mothers' care-seeking for severely sick children. Therefore, GMP+ has the potential to reduce child death in developing countries, i.e., contribute to the Millennium Development Goal 4 aiming at reducing child mortality by two-thirds by 2015.