The aim of this dissertation is to investigate the presence of asymmetric information in the Brazilian health insurance market and the effects that arises due to this asymmetric information: adverse selection and moral hazard. Using the data set of the Health Supplement of the Brazilian Household Sample Survey (Pesquisa Nacional por Amostra de Domicílios - PNAD) for the year 1998, descriptive analysis are produced and statistical inferences are realized to evaluate the moral hazard and adverse selection. In order to have a better perception of the adverse selection problem it is estimated a logistic model to evaluate the relation among the condition of having a health plan and independent variables such as income, age, and health self- perception. In a next step inferences related to moral hazard are realized by two different procedures: the first one, based on the construction of a moral hazard indicator, named MHI, analyses the differences between the number of physician visits for individuals who have and who have not a health plan, considering the sample design of the Health Supplement of the Brazilian Household Sample Survey. The second procedure aims to verify the relation between having a health plan and the number of physician visits by estimating a hurdle negative binomial model, which allows the elimination of some bias in the average number of physician visits.