Our primary objective was to examine the relationship between methamphetamine use and sexual risk-taking behaviors--number of sexual partners, frequency of sexual behaviors with regular and casual partners, trading money or drugs for sex, and condom use--among male and female out-of-treatment injection drug users (OTIDUs). As a risk group for human immunodeficiency virus (HIV) transmission, we also investigated injection behaviors by methamphetamine use. Data were collected from 1392 OTIDUs within the California counties of Fresno, Sacramento, and San Diego. Excluded from this cross-sectional survey were male OTIDUs engaging in sex with only or mostly men since 1978. In bivariate analyses, we found that male OTIDUs with a history of methamphetamine use had more sex partners and participated in more acts of anal insertive intercourse with casual partners and vaginal intercourse with regular and casual partners than male OTIDUs never using methamphetamines. In addition, a greater percentage of male OTIDUs using methamphetamines reported trading sex for money or drugs. Methamphetamine-using female OTIDUs participated in more acts of vaginal intercourse with regular male sex partners than female OTIDUs never using methamphetamines. By multivariate logistic regression, we found methamphetamine use related to consistent condom use among male OTIDUs and among male sex partners of female OTIDUs. Discriminant function analyses revealed that sexual risk taking could be differentiated by methamphetamine use among male OTIDUs. Methamphetamine use also correlated with using shared needles or syringes among male and female OTIDUs and was related to not always disinfecting used needles or syringes with bleach. Our findings suggest that methamphetamines may contribute to heterosexual HIV transmission.