Background\nSubstantial resources have been invested in increasing childhood immunisation coverage through global initiatives such as the Universal Childhood Immunisation (UCI) campaign and the Global Alliance on Vaccines and Immunisations (GAVI). There are longstanding concerns that target-oriented and performance-oriented initiatives such as UCI and GAVI's immunisation services support (ISS) might encourage over-reporting. We estimated the coverage of three doses of diphtheria, tetanus, and pertussis vaccine (DTP3) based on surveys using all available data.\nMethods\nWe estimated DTP3 coverage by analysing unit record data from surveys and supplemented this with reported coverage from other surveys and administrative data. We used bidirectional distance-dependent regression to estimate trends in survey-based coverage in 193 countries during 1986—2006. We used standard time-series cross-sectional analysis to investigate any association in the difference between countries' official reports and survey-based coverage as the dependent variable and the presence of GAVI ISS as the independent variable, controlling for country and time effects.\nFindings\nCrude coverage of DTP3 based on surveys increased from 59% (95% uncertainty interval 51—65) in 1986 to 65% (60—68) in 1990, 70% (65—74) in 2000, and 74% (70—77) in 2006. There were substantial differences between officially reported and survey-based coverage during UCI. GAVI ISS significantly increased the difference between officially reported coverage and survey coverage. Up to 2006, in 51 countries receiving GAVI ISS payments, 7·4 million (5·7 million to 9·2 million) additional children were immunised with DTP3 based on surveys compared with officially reported estimates of 13·9 million. On the basis of the number of additional children immunised from surveys at a rate of US$20 each, GAVI ISS payments are estimated at $150 million (115 million to 184 million) compared with actual disbursements of $290 million.\nInterpretation\nSurvey-based DTP3 immunisation coverage has improved more gradually and not to the level suggested by countries' official reports or WHO and UNICEF estimates. There is an urgent need for independent and contestable monitoring of health indicators in an era of global initiatives that are target-oriented and disburse funds based on performance