To compare potential risk factors for the transition to injecting among non-injecting heroin users (NIUs) with different injecting histories. Cross-sectional data from baseline structured interviews with NIUs in a study on transitions to injecting. Sample recruited by outreach or chain-referral in New York City (NYC), 1996-1998. Recruitment of sample and interviews conducted in a NYC neighborhood where many drug users reside and/or use drugs. Of 575 NIUs, 67% had never injected; 16% had injected one to nine times (infrequent former injectors (IFI)); and 18% 10 or more times (frequent former injectors (FFI)). Controlling for age and race/ethnicity, adjusted odds ratios were estimated in multivariate logistic regression, and differences in means tested by ANCOVA. FFI (compared to never injectors and IFI) were more likely: to be homeless; to be unemployed; to be long-time users; to be younger at first heroin use; to not have initiated heroin use through non-injected routes; to not be afraid of injecting themselves with needles; to sniff heroin with former IDUs; and, for both men and women separately, to have sex partners who were former IDUs. Both FFI and IFI were twice as likely as never injectors to perceive that their friends thought that it was "OK" to inject drugs. FFI have multiple individual and network characteristics that may increase their risk of injecting drugs. Interventions among NIUs to prevent transitions to injecting need to ascertain NIUs' injecting history and address the many potential risks that FFI have for resuming injecting drug use.