The study found no significant increase in the number of persons who had "ever" (lifetime) injected methamphetamine among young IDUs in San Francisco over the 6 years examined. The prevalence of methamphetamine use was high, however, with 50.1 percent of the sample of 1,445 young IDU reporting recent injections. Recent methamphetamine injection (past 30 days) increased significantly, peaking at 60 percent in 2003. Men who engaged in risky sexual behavior (men who had sex with other men injecting drug users) composed the largest group who had ever injected methamphetamine (92.3 percent) as well as those who had injected methamphetamine recently (59.5 percent). For the other groups distinguished in the sample, 81.6 of heterosexual males had ever injected methamphetamine, and 47.3 percent had injected it recently; for female IDUs, 78.4 percent had ever injected methamphetamine, and 46.1 percent recently injected it. These findings suggest that methamphetamine injection was well underway in 1998 among young IDUs, but it did not increase significantly in the years to 2004. Data were obtained from 1,445 young IDUs (under 30 years old) who reported injecting drugs in the previous month. They were recruited with street outreach methods. The current study is a secondary analysis of cross-sectional baseline data collected for a longitudinal study entitled "U Find Out" (UFO). Participants were recruited from 1998 to 2001 and 2003 to 2004. The analysis examined trends in methamphetamine injection over participants' lifetime ("ever") and recently (past 30 days). 2 tables, 2 figures, and 39 references