Private health services have expanded in many developing countries over the last 10Â yr. Qualified private practitioners provide basic health care for poorer groups in urban areas, although health care planners frequently criticize them for providing poor quality of care, charging high fees and failing to provide preventive health advice. In Karachi, a large city with more than 400 slums, private practitioners are important providers of care to the poor. This study assessed the nature and quality of care provided by 201 practitioners selected from four districts of the city. Vignettes of specific medical problems were used to assess their knowledge and their practice was measured by observing 658 doctor-patient contacts. The results show that knowledge was closer to accepted medical management than was their actual prescribing practice. On the other hand, their manners and interpersonal behaviour were good. Thus poor prescribing practice, which might equally stem from market influences as lack of knowledge, is the cause of low standards of care. In these circumstances, didactic in-service training to improve prescribing practice is unlikely to be successful.