BackgroundGlobally 2.5 million children under five years of age die every year due to vaccine preventable diseases. In Tigray Region in Northern Ethiopia, full vaccination coverage in children is low. However, the determinants of defaulting from completion of immunization have not been studied in depth. This study aimed to identify the determinants of defaulting from child immunization completion among children aged 9–23 months in the Laelay Adiabo District, North Ethiopia.MethodsAn unmatched community based case-control study design was conducted among children aged 9–23 months in the Laelay Adiabo District from February—March 2015. A survey was conducted to identify the existence of cases and controls. Two hundred and seventy children aged 9–23 months (90 cases and 180 controls) were recruited from 11 kebeles (the smallest administrative units) by a simple random sampling technique using computer based Open Epi software. Cases were children aged 9–23 months who missed at least one dose of the recommended vaccine. Controls were children aged 9–23 months who had received all recommended vaccines. Data were collected from mothers/care givers using structured pretested questionnaire. The data were entered into Epi Info version 3.5.1 and analyzed using Statistical Package for Social Sciences (SPSS) version 21. Bivariate and Multiple logistic regression analysis were used to identify the predictors of the outcome variable. The degree of association was assessed by using odds ratio with 95% Confidence Interval (CI).ResultThis study shows that mothers who take >30 minutes to reach the vaccination site (Adjusted Odds Ratio (AOR) = 3.56,95%CI:1.58–8.01); households not visited by health extension workers at least monthly (AOR = 2.68,95%CI:1.30–5.51); poor participation in women's developmental groups (AOR = 3.3,95%CI 1.54–7.08); no postnatal care follow-up (AOR = 5.2,95%CI:2.36–11.46); and poor knowledge of child immunization (AOR = 3.3,95%CI:1.87–7.43) were predictors of defaulting from completion of child immunization.ConclusionPostnatal care follow-up, household visits by health extension workers and maternal participation in women’s development groups are important mediums for disseminating information and increasing knowledge to mothers about child immunization. To reduce the rate of defaulters, health providers should motivate and counsel mothers to attend postnatal care. Health extension workers should visit households at least once per month and strengthen mothers’ participation in the women’s development groups.