Methamphetamine use is an increasingly serious public health problem in California and other parts of the country. Despite sensationalistic media attention, however, very little is known about users of this clandestinely consumed drug. Employing methods known as Rapid Assessment and Response, the authors describe the epidemiology and public health implications of methamphetamine use in California's Central Valley, with a focus on Sacramento, which many social indicators suggest has been more severely affected by methamphetamine than any city in the nation. Data sources for this report include interviews with drug users, statistical reports, epidemiologic studies, and local informed expert opinion. In their social demography, methamphetamine users in the Central Valley are in marked contrast to those of coastal cities such as Seattle and San Francisco, being largely heterosexual, and of mixed racial/ethnic heritage. Three-quarters or more initiate their use of the drug while still in their teens, with more than a quarter beginning use before the age of 15. Many of these rapidly gravitate to regular use, and continue using well into their thirties. Methamphetamine users are at much higher risk of infection with HIV than opiate users, particularly if they inject. Partly because methamphetamine enhances libido, users of the drug typically also have many more sexual partners. Not surprisingly, data indicate that methamphetamine users are more likely than heroin users to be HIV-infected. Methamphetamine appears to be less of a street drug than heroin, complicating efforts at street outreach. However, because it is typically used in social settings, a social or diffusion approach to HIV prevention might be particularly promising.