PROBLEM: As national antiretroviral treatment (ART) programmes scale-up, it is essential that information is complete, timely and accurate for site monitoring and national planning. The accuracy and completeness of reports independently compiled by ART facilities, however, is often not known. APPROACH: This study assessed the quality of quarterly aggregate summary data for April to June 2006 compiled and reported by ART facilities ("site report") as compared to the "gold standard" facility summary data compiled independently by the Ministry of Health supervision team ("supervision report"). Completeness and accuracy of key case registration and outcome variables were compared. Data were considered inaccurate if variables from the site reports were missing or differed by more than 5% from the supervision reports. Additionally, we compared the national summaries obtained from the two data sources. LOCAL SETTING: Monitoring and evaluation of Malawi's national ART programme is based on WHO's recommended tools for ART monitoring. It includes one master card for each ART patient and one patient register at each ART facility. Each quarter, sites complete cumulative cohort analyses and teams from the Ministry of Health conduct supervisory visits to all public sector ART sites to ensure the quality of reported data. RELEVANT CHANGES: Most sites had complete case registration and outcome data; however many sites did not report accurate data for several critical data fields, including reason for starting, outcome and regimen. The national summary using the site reports resulted in a 12% undercount in the national total number of persons on first-line treatment. Several facility-level characteristics were associated with data quality. LESSONS LEARNED: While many sites are able to generate complete data summaries, the accuracy of facility reports is not yet adequate for national monitoring. The Ministry of Health and its partners should continue to identify and support interventions such as supportive supervision to build sites' capacity to maintain and compile quality data to ensure that accurate information is available for site monitoring and national planning.