A knowledge, attitude and practice study on vaccination was undertaken among Iranian mothers in pediatric clinic of a university medical center in Tehran, between March and August 2005, and 668 mothers responded to a questionnaire-based interview. The results showed a favorable attitude towards children immunization in 95.5% of respondents. Nearly half (51.4%; 95% confidence interval: 47.6%?55.2%) of mothers knew the name of the diseases against which their children were being vaccinated. More than half (67%) of respondents gave disproportionate importance to mild intercurrent illness as a reason to defer immunization. About half of children (341 = 51.1%) experienced vaccination delay. Results of logistic regression analysis showed increase in: birth order, number of children in household, and mother's age significantly predicted vaccination schedule non-adherence (P = 0.02, P = 0.02 and P = 0.04 to P = 0.001 respectively) and increasing mother's age was the most significant factor for vaccination delay. Educating mothers about the vaccines and vaccine preventable diseases, and improving their performance are recommended.