In 1981, a small group of Brazilian physicists started a very effective campaign to construct a national synchrotron radiation laboratory. By the end of 1984, the project was officially approved and, surviving political shifts brought about by the end of military rule, construction of the lab began in 1987. Why, in these times of declining budgets in world science, should a relatively poor country, struggling with financial difficulties, decide to invest millions of dollars in a Big Science facility? We examine the decision-making process leading to the lab's construction, focusing on three intermediate stages: the decision to build the lab; the choice of its site; and the size of the machine. We show that basic support came much more from policymakers than from scientists and potential users, and that the political ability of the few scientists directly involved with the project was crucial for its implementation. We conclude that the decision to build was made, not to answer scientific problems or to achieve new technological applications relevant to Brazil, but mainly to stimulate technological development, and to introduce the country to the new level of scientific organization represented by Big Science and National Laboratories.282195219