This paper presents an approach to estimating the prevalence of cocain
e use, based upon a new ratio estimation technique. This method can be
applied to random samples of overlapping populations for which no sam
pling frames exist. When the ratio estimation method is applied to the
two study samples (drawn from populations of people using cocaine and
people using heroin) the ratio of cocaine users to heroin users (C/H)
was 1.55, with a 95% confidence interval of +/-0.48. Such estimates s
hould be applied with caution. However, if used with reference to nati
onal estimates of about 75 000 heroin users, application of the presen
t estimate suggests that there may be about 116 000 cocaine users in t
he UK.