Drug users frequently witness the nonfatal and fatal drug overdoses of their peers, but often fail to intervene effectively to reduce morbidity and mortality. We assessed the circumstances of witnessed heroin-related overdoses in New York City (NYC) among a predominantly minority population of drug users. Among 1184 heroin, crack, and cocaine users interviewed between November 2001 and February 2004, 672 (56.8%) had witnessed at least one nonfatal or fatal heroin-related overdose. Of those, 444 (67.7%) reported that they or someone else present called for medical help for the overdose victim at the last witnessed overdose. In multivariable models, the respondent never having had an overdose her/himself and the witnessed overdose occurring in a public place were associated with the likelihood of calling for medical help. Fear of police response was the most commonly cited reason for not calling or delaying before calling for help (52.2%). Attempts to revive the overdose victim through physical stimulation (e.g., applying ice, causing pain) were reported by 59.7% of respondents, while first aid measures were attempted in only 11.9% of events. Efforts to equip drug users to manage overdoses effectively, including training in first aid and the provision of naloxone, and the reduction of police involvement at overdose events may have a substantial impact on overdose-related morbidity and mortality.