This paper studies the trend of immunization dropout and its relationship with economic inequality among Cambodian children aged 12–59 months, using data from the 2000 and 2005 Cambodia Demographic and Health Surveys (CDHS). We define an ‘immunization dropout’ as a child who has received at least one dose of trivalent vaccine against diphtheria, pertussis, and tetanus (DPT), but failed to receive his or her third dose to complete the schedule before 12 months of age. Socioeconomic status is represented by household wealth index quintiles. The level of DPT dropouts from 1996 to 2004 decreased from 22% to 10%. The difference in DPT3 dropouts between children of the highest household wealth quintile and children of the lowest and lower wealth quintiles significantly improved over these years. In 2000, the children of the lowest wealth quintile were almost three times as likely as the children of the highest wealth quintile not to complete the third dose of DPT (Odds Ratios [OR] = 2.92, P = 0.001). In 2005, however, the difference was small and statistically insignificant (OR = 1.42, P = 0.290). These results demonstrated that the child immunization program in Cambodia improved significantly over the past years. The improvement was observed in children of all economic strata, especially among the children of the poorer households.