This study was conducted to evaluate the prevalence of illicit substance use among students of Tehran University of Medical Sciences (TUMS) in four repeated surveys from 2006 to 2009. All first- to fourth-year students of the university were included. The questionnaire was anonymous and self-administered and inquired directly about own personal use. We also inquired about use of substances in close friends present in the class to provide another estimated prevalence. Using self-report, the prevalence of "past-year use of any illicit substance" was 2.3% in 2006, 3.3% in 2007, 2.8% in 2008, and 1.1% in 2009. It did not show a significant linear trend, but showed a significant decline for the year 2009. Hashish, opioids, and amphetamine-type stimulants were the most common types of substances being used. The calculated estimated prevalence rates were higher than those based on self-reports. The results suggest the need for continuous monitoring and appropriate preventive planning.