In studies comparing clinical practice to evidence-based standards, researchers have found that quality of care is inconsistently provided to different segments of the population in both developing and developed countries. To test the hypothesis that quality of care varies widely within different countries, we conducted a prospectively designed evaluation of quality for three common clinical conditions: diarrhoea, tuberculosis and prenatal care. Five countries participated in the study: China, the Philippines, Mexico, El Salvador and India. Within each country, physicians were randomly selected from tertiary care hospitals, district level hospitals, and public and private outpatient clinics. A total of 488 previously validated case vignettes were administered to 300 participating physicians. Vignettes were scored according to evidence and expert based quality criteria. We used a random effects model to estimate the associations between quality scores by case, physician characteristics, study site, and country. We found that average quality of care was low (61.0%), but there exists a wide variation in overall quality (30-93%). While there was little difference in average quality scores between countries (60.2 to 62.6%), variation within countries was broad. The wide variation was consistent across facility type, medical condition and domain of care. We also found that younger, female, tertiary care and specialist physicians performed better than their counterparts. We conclude that some physicians provide exceptional care even in the setting of limited resources. Furthermore, poor quality can be addressed by health policy planners by directing remediation toward the lowest performers.