The widely-used anti-convulsant and anxiolytic drug clonazepam (Klonopin^®) requires monitoring, as the drug is subject to both abuse and diversion. Urine is the most-used and convenient specimen for monitoring benzodiazepine use in the general population. Urine benzodiazepine immunoassays detect clonazepam use very poorly however, as they are generally insensitive at detecting 7-aminoclonazepam-related species, the clonazepam derivatives that predominate in urine. Simultaneously collected urine-serum pairs were obtained from 106 patients taking clonazepam and judged to be free of other benzodiazepines. The authors tested these specimens using three urine benzodiazepine immunoassays: the Roche Kinetic Interaction of Microparticles in Solution (KIMS), Syva EMIT II+, and Biosite Triage. The KIMS assay was tested using three different cut-offs: 100, 200, and 300 ng/mL. The KIMS assay detects the primary urinary metabolite 7-aminoclonazepam with equal sensitivity as the parent drug, while the others do not. Analysis of 55 urine samples from confirmed clonazepam users free of other benzodiazepines, yielded the following sensitivities for detecting clonazepam use: KIMS (100 ng/mL cutoff), 82%; KIMS (200 ng/mL cutoff), 56%; KIMS (300 ng/mL cutoff), 36%; Syva EMIT II+ (200 ng/mL cutoff), 27%; Biosite Triage (300 ng/mL cutoff), 18%. This order of sensitivity matched the sensitivity of each assay’s ability to detect clonazepam’s major urinary metabolite, 7-aminoclonazepam. Excluding dilute urines (creatinine < 0.50 mg/mL) KIMS100 was 96% (87 of 91) sensitive at detecting clonazepam use. Urine creatinine concentration was more useful than serum clonazepam concentration in rationalizing a patient’s urine benzodiazepine immunoassay results. The KIMS100 sensitivity, 96%, far exceeds that of the other urine benzodiazepine immunoassays studied here, rivaling LC-MS-MS at estimating patient compliance with clonazepam use.