A representative sample of 354 male heroin addicts living in the Baltimore metropolitan area was traced from onset of opiate use to time of interview to ascertain any changes in the frequency or type of offences committed during their years at risk. Five basic measures of criminality were employed: crime-day theft, crime-day violence, crime-day dealing, crime-day con games and crime-day other offences. A sixth measure -- composite crime day -- incorporated all five crime-day measures. Crime rates per year were derived from these six measures. It was found that the start of addiction was associated with a high level of criminality (255 composite crime-days per year), and that this high rate continued over numerous subsequent periods of addiction. Theft of property was the most common type of crime, followed by drug sales, other offences, con games, and violent offences. In contrast to the addiction periods, criminality decreased over successive non-addiction periods. Thus, the composite crime rate (82 composite crime-days per year) for the first non-addiction period was only 32% of the rate of the first addiction period and this lower rate of criminality decreased markedly thereafter.