Injecting drug users who share equipment may transmit and acquire bloodborne virus infections, including HIV, hepatitis B virus, and hepatitis C virus. Even without sharing, injection with non-sterile equipment, drugs, or mixing agents may result in infection due to bacteria or fungi. Estimates of the number of people who are currently at risk of infection from injecting drug use are needed in order to plan services and care, and to interpret surveillance data. This paper examines the data from registries of drug use and two recent surveys of the general population from which estimates of the number of current injecting drug users in England and Wales have been derived. Drug registries include only those whose drug use is identified during contact with drug or medical services, so these sources provide minimum estimates but may be used to monitor trends: 25,706 drug users in England and Wales were notified to the Home Office in 1993, 12,253 of whom were current injectors. Estimates derived from surveys of the general population suggest, however, that between 51,900 (95% confidence interval (CI): 33,000-71,600) and 77,700 (95% CI: 4100-151,200) people in England and Wales are at risk of infection from current injecting drug use, of whom between 10,400 (95% CI: 7200-13,800) and 15,500 (95% CI: 800-30,200) are at risk of bloodborne virus infections as a result of sharing injecting equipment. In the 16 to 34 year age group about one in 200 men, and one in 400 to 500 women may be current injectors.(ABSTRACT TRUNCATED AT 250 WORDS)