In a case-control analysis of cross-sectional data, 328 children aged 12-35 months and their mothers were studied to identify the factors associated with delayed or non-immunization of their children. Delayed or non-immunization was associated with low socio-economic status, maternal illiteracy, and lack of mothers' knowledge on vaccine preventable diseases as recommended by the Expanded Programme on Immunization (EPI). The association of this lack of mother's knowledge with no or delayed immunisation persisted after adjusting the effects of others in logistic regression analysis (Odds Ratio 16.7; 95 per cent confidence interval: 15.65-17.8; P < 0.0001). The results indicate that even in the presence of maternal illiteracy, educating mothers about the vaccines and vaccine preventable diseases may be highly effective in increasing the immunization coverage.