Tetrahydrocannabinol (THC) is the main psychoactive ingredient in cannabis. The study found that the THC content of the cannabis in England varied widely, as did the content of other cannabinoids, especially in herbal cannabis (marijuana) and cannabis resin (hashish). The average potency of cannabis within the country is apparently increasing, but large variation remains within and between various areas of the country. The median THC content of herbal cannabis and resin was 2.1 percent and 3.5 percent, respectively. In sinsemilla and imported herbal cannabis, the content of the antipsychotic cannabinoid cannabidiol (CBD) was extremely low. In resin, however, the average CBD content exceeded that of THC, and the relative proportions of the two cannabinoids varied widely among samples. The increases in average THC content and relative popularity of sinsemilla cannabis, combined with the absence of the antipsychotic cannabinoid CBD, suggest that the current trends in cannabis use pose an increasing risk to users who are susceptible to the harmful psychological effects associated with high doses of THC. Samples were collected from the police property stores of five constabularies. All samples were collected during a range of dates in 2005, some of which had been stored at room temperature for up to a year. The majority of the samples had been seized from users and suppliers "on the street." The remainder were found during searches of property suspected of being involved in cannabis production or consumption. The samples were assessed visually, using a simple light microscope where necessary, and the form of the cannabis samples was established. The analysis method was that developed by de Meijer et al. for the validated identification and quantification of a range of cannabinoids. 1 table, 1 figure, and 27 references