The hidden drug abuse in a stratified sample of a year cohort born in 1953 was studied by measuring the difference between drug abuse stated in interviews and registered in public health and social welfare files in 1968, 1973 and 1976. Among men who had stated high-frequency drug use in a school questionnaire in 1968 hidden drug abuse comprised two thirds of the total abuse, among women from the same group one half. In groups with lower degrees of abuse hidden drug abuse was 70-90% of the total abuse. Intravenous abuse was mostly known to public health and social welfare authorities. When trying to estimate the total number of drug abusers in an area there is reason to at least double the figures presented in case-finding studies.