An investigation was carried out as a case-finding study to estimate the scale of heavy drug abuse in Sweden. Just over 8200 persons were reported as heavy drug abusers, 80% of these as injecting. For the majority of those reported more than one type of drug was indicated. There was concurrent abuse of alcohol by a majority of the heavy drug abusers. After correction of non-response with a modified capture-recapture technique and for erroneous classification, the scale of heavy drug abuse was estimated at 10000 - 14000 persons.