The Severity of Dependence Scale (SDS) was devised to provide a short, easily administered scale which can be used to measure the degree of dependence experienced by users of different types of drugs. The SDS contains five items, all of which are explicitly concerned with psychological components of dependence. These items are specifically concerned with impaired control over drug taking and with preoccupation and anxieties about drug use. The SDS was given to five samples of drug users in London and Sydney. The samples comprised users of heroin and users of cocaine in London, and users of amphetamines and methadone maintenance patients in Sydney. The SDS satisfies a number of criteria which indicate its suitability as a measure of dependence. All SDS items load significantly with a single factor, and the total SDS score was extremely highly correlated with the single factor score. The SDS score is related to behavioural patterns of drug taking that are, in themselves, indicators of dependence, such as dose, frequency of use, duration of use, daily use and degree of contact with other drug users; it also shows criterion validity in that drug users who have sought treatment at specialist and non-specialist agencies for drug problems have higher SDS scores than non-treatment samples. The psychometric properties of the scale were good in all five samples, despite being applied to primary users of different classes of drug, using different recruitment procedures in different cities in different countries.