Most primary health care services in developing countries are delivered by staff working in peripheral facilities where supervision is problematic. This study examined whether systematic supervision using an objective set of indicators could improve health worker performance. A checklist was developed by the Philippine Department of Health which assigned a score from 0 to 3 on each of 20 indicators which were clearly defined. The checklist was implemented in 4 remote provinces with 6 provinces from the same regions serving as a control area. In all 10 provinces, health facilities were randomly selected and surveyed before implementation of the checklist and again 6 months later. Performance, as measured by the combined scores on the 20 indicators, improved 42% (95% Cl = 29% to 55%) in the experimental group compared to 18% (95% Cl = 9% to 27%) in the control group. In the experimental, but not in the control facilities, there was a correlation between frequency of supervision and improvements in scores. The initial cost of implementing the checklist was US $ 19.92 per health facility and the annual recurrent costs were estimated at $ 1.85. Systematic supervision using clearly defined and quantifiable indicators can improve service delivery considerably, at modest cost.