With the increasing evidence for the significant contribution to violence made by alcohol and drug misuse in people with mental disorder, there is emerging evidence of the importance and increasing prevalence of substance misuse problems in patients admitted to high security hospitals in the UK. Recorded routine voluntary urine drug screening of patients at the State Hospital, the high security hospital for Scotland and Northern Ireland, started in 1998. Results of urinalysis over the 3 years from 1 January 1999 to 31 December 2001 were examined. Out of 7,069 samples sent from 421 patients there were 712 (10.1%) positive results. The majority were false positives (672; 9.5%) due to prescribed medication, with only 40 (0.6%) true positives. All the true positive results in the admission unit were related to substance misuse prior to admission, the remaining true positive results were all within the continuing care wards, with no true positives in the rehabilitation wards. Even though the testing was not truly random, it is likely that the findings reflect extremely low levels of substance misuse within the hospital in contrast with findings from UK prisons. Whether urine testing contributes to the prevention of drug misuse within the hospital is unclear, and there are potential ethical and legal concerns about the urine testing process. It is important that high security hospitals are virtually drug free, to enable the uncontaminated assessment and treatment of mentally disordered offenders.