This work is aimed at providing improved coefficients applicable to the statistics of Health insurance concerning the long-lasting conditions in order to assess morbidity of Health Insurance members concerning diseases invoked legislation on long-lasting conditions. To reach this objective, we have applied the consequences of successive equivalences (between diseases and their specific treatment, and between the use of special prescriptions for exempting diseases and the acknowledgment of long-lasting conditions by Health Insurance) to the results of a survey on medicines carried out by the Caisse Nationale d'Assurance Maladie des Travailleurs Salarie's in 1993 on the basis of a representative sample. Improved coefficients were equal to the inverse of the proportion of special prescriptions within the whole of the prescriptions including specific treatment. Those coefficients and their confidence intervals have been calculated for seven affections : non-insulin dependent diabetes [1.56-1.67], coronary disease [1.51-1.61], Parkinson's disease [1.51-1.85], manic-depressive psychosis [1.49-1.89], haemorrhagic rectocolitis and Crohn's disease [1.49-2.04], insulin dependent diabetes [1.10-1.20] and tuberculosis [1.61-3.33]. The critical analysis of our work and its comparison with the rare literature on the same subject confirms the value of our results. The application of those improved coefficients to the figures of prevalence and incidence provided by Health Insurance makes it possible to set confidence bounds to the statistics of morbidity concerning the people who are affiliated to that system of Health insurance and treated for these diseases however serious they are.