A Swedish web-based service (www.escreen.se) offers self-assessment and self-monitoring of alcohol and drug use via on-line screening with the Alcohol Use Disorders Identification Test (AUDIT) and the Drug Use Disorders Identification Test (DUDIT) as well as in-depth risk assessment using extended versions of both tests (Alcohol-E and DUDIT-E). Users receive individualized feedback concerning their alcohol and drug consumption and can follow their alcohol and drug use over time in personal diagrams and by writing in an electronic diary. This study describes user characteristics, service utilization patterns, and psychometric test properties for 2361 individuals who created a valid account over 20 months starting in February 2007. Problematic alcohol use according to AUDIT criteria was indicated for 67.4%, while 46.0% met DUDIT criteria for problematic drug use. Men and women accessed the service equally, with a mean age of 23 years. Internal consistency reliability figures were 0.90 for 1846 first-time AUDIT users and 0.97 for 1211 first-time DUDIT users; among 213 second-time AUDIT users reliability was 0.93, and 0.96 for 97 second-time DUDIT users. Internet-based alcohol and drug monitoring could function as a self-help tool or as a complement to substance abuse treatment.