Abstract: The number of injecting drug users in the Central and Eastern Sydney Area Health Services (CSAHS and ESAHS) in 1989–1990 was estimated by applying the Petersen mark-recapture method to data obtained from the Australian National AIDS and Injecting Drug Use Surveys conducted in 1989 and in 1990. The population estimates for injecting drug users were 8368 (95 per cent confidence interval (CI) 6099 to 11 829) for ESAHS and 1466 (CI 742 to 2841) for CSHS. An estimate was also obtained for the number of injecting opioid users residing within the ESAHS; our figure of 3597 (CI 2731 to 5737) was consistent with previously reported estimates obtained using mark-recapture but based on different data sets.