In a 6-year period, 435 alleged cannabis samples were accessioned by an urban street drug analysis program, and clinical histories were correlated with laboratory results. The most common reason for submission of a sample was suspicion of adulteration; however, such samples were usually unadulterated but had high potency as measured by delta 9-tetrahydrocannabinol concentration. Adverse reactions occurring in nonadulterated samples were not associated with high potencies. The validity of cannabis products distributed illicitly and analyzed was over 94%. Exotic varieties were more potent than less expensive cannabis. The absolute amount of delta 9-tetrahydrocannabinol ranged from 1.5 to 144.9 mg.