The diffusion and trends in use of each substance is a basic information in policy planning of strategies aiming at deterrence of drug abuse or in the organization of the fight against drug trafficking. The actual diffusion of illicit drugs in a population is hardly measurable, but, among the various measures available, the analysis of waste water plants represents one of the most reliable source of data. We analyzed waste water in order to monitor illicit drug use by local population. We investigated the use of cocaine and heroin in the city of Florence, Italy, over a 1-year (July 2006-June 2007) period using state-of-the-art measuring techniques from waste water samples. Cocaine, benzoylecgonine, and morphine were determined in water samples by gas chromatography-mass spectrometer, and the amount of illicit substance was estimated. Data indicate for cocaine a bimodal distribution (December and March), while heroin showed a main peak in April. The heroin-to-cocaine use ratio in terms of estimated doses per month ranged from 0.11 to 0.76, representing new evidence of wider distribution of cocaine than heroin in Florence. Waste water analysis can become a valuable tool in monitoring use of illicit drugs over time. In particular, it can highlight changes in the magnitude and relative use of illicit drug at a population level thereby becoming useful to develop strategies against drug trafficking and abuse. If routinely performed, it can be part of Epidemiologic Surveillance Programmes on drug abuse.