This research sought to inform three questions of relevance to illicit drug policy: what effect does the perceived price, purity and availability of heroin have on (a) heroin use and (b) heroin expenditure; what effect does the perceived risk of scoring, perceived hassle associated with scoring and amount of contact with police have on (a) heroin use and (b) heroin expenditure; and what differences are there in terms of adverse health and behavioural outcomes between IDUs who use heroin only and IDUs who use a combination of heroin and psycho-stimulant drugs, such as cocaine and methamphetamine? To address these questions a sample of 296 Sydney IDUs were surveyed. For this sample of IDUs, most of the law enforcement-related variables exerted no significant effect on heroin use and expenditure. Neither the cost of heroin, nor its purity, nor the perceived risk and hassle associated with purchasing heroin were related to either drug expenditure or use. There was a significant relationship between the amount of contact with police and heroin expenditure but it was in the opposite direction to that which would be expected if police contact directly reduces heroin expenditure. IDUs who had more contact with police spent more on heroin each week than IDUs who had relatively infrequent contact with police. The only law enforcement-related variable that appeared to be related to heroin use and expenditure in the manner expected was time to score. IDUs who took longer to score spent significantly less on the drug, and used significantly less of it. (Conclusion, edited)