To improve uptake of childhood immunizations in Wandsworth we developed a standardized call/recall system based on parents being sent three reminders and defaulters being referred to a Health Visitor. Thirty-two out of 44 primary care practices in the area implemented the intervention in September 2011. The aim of this study was to evaluate the implementation, delivery and impact on immunization uptake of the new call/recall system. To assess implementation and delivery, a mixed method approach was used including qualitative (structured interviews) and quantitative (data collected at three months post-implementation) assessment. To assess the impact, we used Student's t test to compare the difference in immunization uptake rates between intervention and non-intervention practices before and after implementation. The call/recall system was viewed positively by both parents and staff. Most children due or overdue immunizations were successfully captured by the 1st invitation reminder. After three invitations, between 87.3 % (MMR1) and 92.2 % (pre-school booster) of children identified as due or overdue immunizations successfully responded. Prior to implementation there was no difference in uptake rates between intervention and non-intervention practices. Post-implementation uptake rates for DTaP/IPV/Hib, MMR1, MMR2 and the pre-school booster were significantly greater in the intervention practices. Similar findings were seen for PCV and Hib/MenC boosters, although the differences were not statistically significant at the 5 % level. Following the successful implementation of a standardized call/recall system in Wandsworth, other regions or primary care practices may wish to consider introducing a similar system to help improve their immunization coverage levels.