Truncated models are indirect methods to estimate the size of a hidden population which, in contrast to the capture-recapture method. can be used on a single information Source. We estimated the coverage of a tuberculosis screening programme among illicit drug users and homeless persons with a mobile digital X-ray unit between 1 January 2003 and 31 December 2005 in Rotterdam. The Netherlands, using, truncated models. The screening programme reached about two-third of the estimated target Population at least once annually. The intended coverage (at least two chest X-rays per person per year) was about 23%. We conclude that simple truncated models can be used relatively easily oil available single-source routine data to estimate the size of a population of illicit drug users and homeless persons. We assumed that the most likely overall bias in this Study Would be overestimation and therefore the coverage of the targeted mobile tuberculosis screening programme would be higher.