Illicit drug injection typically occurs in private or semi-public settings where two or more injectors are present. In a large sample of young adult injectors (aged 15-30) in five US cities, we describe those who reported consistently injecting by themselves in a recent period. Among 3199 eligible subjects, 85% were male, median age was 24 years, and median number of years injecting was four. Fifteen percent (n=467) who reported always injecting alone in the previous 3 months were compared to other IDUs to understand the relationship between this practice and injection risk behavior. IDUs who reported injecting alone were substantially less likely to report injection with a syringe (AOR=0.16, 95% CI 0.1-0.2) or other drug preparation equipment (AOR=0.17, 95% CI 0.13-0.2) previously used by another injector. Markedly low rates of injection risk behavior were observed in IDUs who reported injecting alone; this practice may facilitate safe injection by granting the individual greater control over the injection setting. However, risks may include accidental overdose with severe consequences.