A rapid and simple method was optimized for determination of Δ9-tetrahydrocannabinol (Δ9-THC), cannabidiol (CBD), and cannabinol (CBN) contents in cannabis products by gas chromatography with flame-ionization detection (GC-FID), using diazepam as internal standard. All parameters of validation of the method such as linearity, intraassay precision, and limits of detection and quantification of the analytes were satisfactory. Using the described method, cannabinoid contents of 55 cannabis product samples seized in Sao Paulo City, Brazil, in 2006 and 2007 were measured. Δ9-THC content in marijuana and hashish samples varied between 0.08% and 5.5%, with an average of 2.5%. The phenotypic ratio showed that the products were able to be designated as “drug type.”