An investigation was conducted in southern Tanzania on East Coast fever (ECF) immunization using Infection and Treatment Method (ITM). Two groups, each with 768 cattle were randomly allocated from study farms. One group was subjected to ITM while the second to acaricide application alone. ECF cases occurrence among study animals and calves born to them were investigated. ECF immunization, treatment records and herd dynamics were investigated from both farmers and livestock personnel in addition to conducting structured interviews. Samples were collected from suspect ECF cases and subjected to microscopy. Retrospectively, 2795 cattle were immunized against ECF in the years 2000 to 2009; whereas, during prospective investigation, 768 cattle were immunized. Questionnaire survey revealed that majority of the respondents 154/156 appreciated the protection conferred to immunized animals. However, a good number of them (135/156) complained that calves born to immunized cows succumb to ECF early in life unlike calves born to non-immunized animals. All interviewed livestock personnel (n=12) reported occurrence of ECF in majority of calves born to ECF vaccinated cows; and that most of the cases were detected in calves below ten days. Livestock personnel further pointed out that the response of such infected calves to treatment was poor. According to them, recovery rates of treated calves born from ECF immunized cows ranged from 21 to 60%. Five hundred and fifteen suspect ECF cases occurred in study groups during this period. Majority of the cases (431/515) were calves born to immunized cows, 247/431 being below ten days of age. Some (320/515) of the suspect cases of ECF were subjected to microscopy following Giemsa staining and 313/320 were confirmed by detection of Koch’s blue bodies (KBBs) in lymph smears. Recovery rate of ECF cases in calves born to ECF immunized cows was 46.6% for those below ten days. The rate however increased with age to 100.0%. ECF calves born to non-immunized cows had a recovery rate ranging from 80.0 to 94.7% with the recovery rate increasing with age. In conclusion, ITM of the live Theileria parva confers a reasonable protection against clinical ECF to cattle. The method, however, results in new born calf losses due to Theileria parva infections originating from immunized cows irrespective of vaccine lot numbers. The vaccine thus needs further verification on its safety to the foetuses and non-immunized animals that are kept together with immunized animals.