This article analyses the impact of public investment on private investment. Apart from purely ideological aspects, two opposing interpretations may be distinguished with regard to the relationship between these variables. The first is that there is competition between public and private investment, so that the former "crowds out" the latter. The second is that public investment is complementary to private investment in so far that, by generating positive externalities, it creates favourable conditions for the latter. In view of the relative scarcity of empirical studies on this matter, this study deals with the case of the Brazilian economy in the period from 1947 to 1990. Its main conclusions are that private investment is indeed crowded out by public investment in the short term, but in the long term the coefficients of the cointegration vector indicate that these two variables complement each other.