This study was designed to assess the frequency of exclusive breastfeeding in two health centers of Shahid Beheshti University of Medical sciences in Khak Sefid, Tehran, Iran. A total of 175 mothers, referred for the third dose of their infants' DPT vaccination program participated in the study by completing a questionnaire regarding characteristics of their pregnancy, delivery and exclusive breastfeeding within the first six months of birth. Two-variable analysis and logistic regression test were applied to evaluate factors influencing exclusive breastfeeding. Results indicated that the frequency of exclusive breastfeeding, i.e., breastfeeding within the first six months of birth without the use of any other food with or without vitamin supplementation, was 31.17% (95% CI=23.77%-38.57%), which means 48 infants of 154<179 days old Among 154 infants (<179 days old) 48 did not have a history of being separated from their mothers. In logistic regression analysis, the variables which were directly associated with exclusive breastfeeding, with 0.05 significance level of alpha, included breastfeeding within the first hour of birth, eight times or more breastfeeding per day and receiving breastfeeding education during pregnancy. Variables with a negative association with breastfeeding included lack of breast milk, presence of a breast problem that could hinder breastfeeding, bottle feeding, physician or family's advice not to breastfeed and infant's refusal to breastfeed. Frequency of breastfeeding within the six months of birth is less than similar frequencies which are obtained by asking about breastfeeding on the day of the interview. It is recommended to apply real frequency for assessment, evaluation and programming of exclusive breastfeeding during the first six months of birth.