The development of operational health information systems in sub-Saharan African countries still poses some difficult challenges. In fact, the analysis of the 1996 annual report from the « Health for All Project » in Kinshasa contains a certain number of ambiguities. They are situated at the levels of indicators for coverage which exceed 100% and of incidence rates equal to 0%, whereas they’re were certain cases of poor choices for the denominator. This article discusses the ensuing difficulties of interpretation and suggests presenting the coverage of the programme for the target population separately from that of the population outside the health zone, as well as proposing to give the constant K the value of 1000 or 10000 if needed in order to attain the true incidence rate, which otherwise would remain equal to zero. The correct denominator is proposed where it is indicated.