The aim of epidemiological monitoring is to describe, as realistically as possible, the magnitude, evolution and distribution of a disease or health problem affecting a given population. In most instances this goal is not fully attainable, because of shortcomings of greater or lesser importance affecting the completeness and validity of systems that may themselves vary, not only as between populations but also over time. In practice, a surveillance system can be deemed satisfactory, if it furnishes information of sufficient quality to contribute to timely and appropriate decision making for disease control purposes.

AIDS case reporting systems have played a fundamental part in the fight against this disease. For some time, such systems were practically the only source of information in this epidemic and, after the introduction of HIV epidemiological surveillance activities, have continued to play an important part. Case reporting information has made it possible to: ascertain the number of persons developing AIDS; make indirect estimates of the number of HIV infected persons; and describe the most frequent transmission mechanisms in the respective areas, the characteristics of the AIDS affected population, the geographical differences in the epidemic between countries and regions, and the changes in all these characteristics over time.1

Satisfactory performance of such functions depends on the reporting completeness. Places whose systems evince important shortfalls in completeness may well be operating on the basis of a distorted picture of the reality of the local epidemic, except of course in those cases where these shortfalls have been assessed and duly taken into account on …