A national network including 114 blood banks and laboratories performs screening for HIV infection on approximately 250,000 blood donors a year. This represents 100% coverage of blood transfusions performed in Chile. Up to December 1990. 4956 samples had been sent to the National Reference Center for AIDS for supplementary studies. HIV infection was confirmed in 1465 (29.6%). A prior study covering the first semester of 1989 showed a 26.9% confirmation rate for HIV infection on 845 serum samples. Technical errors including sample contamination, poor cleaning, decreasing titers of controls and erratic criteria regarding the need for supplementary studies may explain the low confirmation rate reported in this study.