The present invention relates to the use of a combination of two different cannabinoids in the treatment of leukaemia. The combination of CBD with THC appears to be particularly effective in reducing cell number in this disease. Preferably the cannabinoids are used in the form of an extract of Cannabis such that many of the naturally occurring compounds are co-extracted with the THC or CBD. Alternatively, the cannabinoids are present in the form of a highly purified extract of Cannabis, wherein the CBD or THC are present at greater than 98% of the total extract (w/w) and the other components of the extract are characterised. Alternatively, the CBD and THC may be synthetically produced. A specific ratio of CBD and THC such as 10:1 to 1:10 (CBD:THC) or more preferably between 2:1 to 1:2 (CBD:THC) may be used.