Various studies have reported that abuse of cannabis is a risk factor for psychosis. The aims of this study were to determine the prevalence of delta 9-tetrahydrocanabinol (Δ(9)-THC), a major metabolite of cannabis, in psychiatric patients in Uganda, and to assess the diagnostic capacity of two referral mental health hospitals to screen patients for exposure to cannabis in Uganda. Socio-demographic characteristics of the patients were collected through questionnaires and review of medical records. Urine samples were collected from 100 patients and analyzed using Δ(9)-THC immunochromatographic kit (Standard Diagnostics(®), South Korea). Seventeen percent of the patients tested positive for Δ(9)-THC residues in their urine. There was strong association (P < 0.05) between history of previous abuse of cannabis and presence of Δ(9)-THC residues in the urine. Alcohol, cocaine, heroin, pethidine, tobacco, khat and kuber were the other substances abused in various combinations. Both referral hospitals lacked laboratory diagnostic kits for detection of cannabis in psychiatric patients. In conclusion, previous abuse of cannabis is associated with occurrence of the residues in psychiatric patients, yet referral mental health facilities in Uganda do not have the appropriate diagnostic kits for detection of cannabis residues as a basis for evidence-based psychotherapy.