OBJECTIVE: To assess the quality of medical treatment by disaggregating quality into components that distinguish between insufficient and unnecessary care. DESIGN: Randomly selected doctors were asked how they would treat a sick child. Their responses were disaggregated into how much of an evidence-based essential treatment plan was completed and the number of additional non-essential treatments that were given. Key variables included the expected cost, the health consequences of insufficient and unnecessary care and comparisons between public and private physicians. Responses to 160 clinical performance vignettes (CPVs) were analysed. SETTING: Philippines. PARTICIPANTS: One hundred and forty-three public and private physicians in the Philippines, collected in November 2003-December 2004 and September 2006-June 2007. INTERVENTIONS: CPVs administered to physicians. MAIN OUTCOME MEASURES: Process quality measures (accounting for the possibility of both over-treatment and under-treatment). RESULTS: Based on CPVs, doctors gave both insufficient and unnecessary treatment to under-five children in 69% of cases. Doctors who provided the least sufficient care were also the most likely to give costly or harmful unnecessary care. Insufficient care typically had potentially worse health consequences for the patient than unnecessary care, though unnecessary care remains a concern because of overuse of antibiotics (47%) and unnecessary hospitalization (34%). CONCLUSIONS: Quality of care is complex, but over- and under-treatment coexist and, in our analysis physicians that were more likely to under-treat a sick child were also those more likely to over-treat.