Bayesian capture-recapture analysis was undertaken to estimate the number and demogrpahy of injecting drug users (IDUs) in Scotland using data from 2006. Individual level data were collected using four (non-independent) data-sources - social enquiry reports (901 IDUs listed), hospital records (953), drug treatment agencies (3504) and recent Hepatitis C virus diagnoses (827) - so that individuals could be cross-classified by the sources which listed them. Individual-level demographic information was also available on gender, age-group (<35, 35+) and geographical location (Greater Glasgow and Clyde, rest of Scotland). The5670 unique listed individuals are a lower bound on Scotland's number of IDUs, as the estimated numbed not identified by any of the four data-sources needs to be added. We modelled the observed data using standard hierarchical log-linear models to allow for interactions between pairs of data-sources and/or demographic classifications. The model-averaged mean, which combines uncertainty about both parameters and model, was 31700 IDUs to nearest 100 (95% credible interval: 24800 to 38700). Based on drugs-related deaths (DRDs) in 2006+2007, it was only in the younger age-group that Scotland's opiate-related DRDs per 100 IDUs were significantly fewer for females than males. Irrespective of region, older males' opiate-related DRD rate was estimated at 1.3 (0.95 to 1.65) per 100 IDUs.