Ketamine, a "club drug" commonly administered intranasally among youths for its disassociative properties, has emerged as a drug increasingly common among a new hidden population of injection drug users. Because of a scarcity of epidemiological data, little is known about ketamine injection practices, associated risk behaviors, or the demographic characteristics of ketamine injectors. Using an ethno-epidemiological methodology, we interviewed 40 young (<25 years old) ketamine injectors in New York during 2000-2002 and asked detailed questions about ketamine injection initiation as well as histories of other injection drug use and involvement in the street economy. Our analysis, utilizing descriptive statistics and narrative accounts, compared two groups: ketamine initiates (youths who initiated injection drug use with ketamine) and other initiates (youths who initiated injection drug use with another drug, such as heroin, and later transitioned into ketamine injection). Results indicated that intramuscular injections were more common among ketamine initiates, whereas intravenous injections were more common among other initiates. Drug form and local knowledge within injection groups were important factors underpinning this relationship: liquid ketamine was injected primarily intramuscularly; powder ketamine was injected primarily intravenously virtually irrespective of injection drug use history. In addition, the comparison between ketamine initiates and other initiates revealed differences regarding knowledge about injecting drugs; risk behaviors at initiation; involvement in the street economy, including homelessness and experience dealing drugs; and city or location of ketamine injection initiation. These findings suggest that ketamine injection is an emerging practice among a new hidden population of injection drug users in cities throughout North America.