Urban metabolism studies have been established for only a few cities worldwide, and difficulties obtaining adequate statistical data are universal. Constraints and peculiarities call for innovative methods to quantify the materials entering and leaving city boundaries. Such methods include the extrapolation of data at the country or the region level based, namely, on sales, population, commuters, workers, and waste produced. The work described in this article offers a new methodology developed specifically for quantifying urban material flows, making possible the regular compilation of data pertinent to the characterization of a city's metabolism. This methodology was tested in a case study that characterized the urban metabolism of the city of Lisbon by quantifying Lisbon's material balance for 2004. With this aim, four variables were characterized and linked to material flows associated with the city: absolute consumption of materials/products per category, throughput of materials in the urban system per material category, material intensity of economic activities, and waste flows per treatment technology. Results show that annual material consumption in Lisbon totals 11.223 million tonnes (20 tonnes per capita), and material outputs sum 2.149 million tonnes. Nonrenewable resources represent almost 80% of the total material consumption, and renewables consumption (biomass) constitutes only 18% of the total consumption. The remaining portion is made up of nonspecified materials. A seemingly excessive consumption amount of nonrenewable materials compared to renewables may be the result of a large investment in building construction and a significant shift toward private car traveling, to the detriment of public transportation.