BACKGROUND: Urine drug tests (UDTs) are recommended for patients on chronic opioid therapy (COT). Knowledge of the risk factors for aberrant UDT results could help optimize their use. OBJECTIVE: To identify primary care COT patient and opioid regimen characteristics associated with aberrant UDT results. DESIGN: Population-based observational. SAMPLE: 5,420 UDTs for Group Health integrated group practice COT patients. MEASURES: Group Health database measures of patient demographics, medical history, COT characteristics, and UDT results. RESULTS: Thirty percent of UDTs had aberrant results, including prescribed opioid non-detection (12.3 %), tetrahydrocannabinol (THC; 11.2 %), non-prescribed opioid (5.3 %), illicit drug (excluding THC; 0.6 %), nonprescribed benzodiazepine (1.7 %), and dilute (4.8 %). Adjusted odds ratios (95 % CI) of any aberrant result were higher for males than females CONCLUSIONS: In this primary care setting, results were aberrant for 30 % of UDTs of COT patients, largely because of prescribed opioid non-detection and THC. Aberrant results of almost all types were more likely among patients under the age of 45. Other risk factors varied across aberrancies, but commonly included current smoking and prior substance use disorder diagnosis.KEY WORDS: urine drug test; chronic opioid therapy; chronic pain; marijuana.