Grievous bodily harm means really serious bodily harm. It is for the jury to decide whether the harm is really serious. However, examples of what would usually amount to really serious harm include:
* injury resulting in permanent disability, loss of sensory function or visible disfigurement;
* broken or displaced limbs or bones, including fractured skull, compound fractures, broken cheek bone, jaw, ribs, etc;
injuries which cause substantial loss of blood, usually necessitating a transfusion or result in lengthy treatment or incapacity;
* serious psychiatric injury. As with assault occasioning actual bodily harm, appropriate expert evidence is essential to prove the injury
In accordance with the recommendation in R v McCready (1978) 1 WLR 1376, if there is any reliable evidence that a sufficiently serious wound has been inflicted, then the charge under section 20 should be of unlawful wounding, rather than of inflicting grievous bodily harm. Where both a wound and grievous bodily harm have been inflicted, discretion should be used in choosing which part of section 20 more appropriately reflects the true nature of the offence.
The prosecution must prove under section 20 that either the defendant intended, or actually foresaw, that the act might cause some harm. It is not necessary to prove that the defendant either intended or foresaw that the unlawful act might cause physical harm of the gravity described in section 20. It is enough that the defendant foresaw some physical harm to some person, albeit of a minor character, might result: (R v Savage; DPP v Parmenter  1 A.C 699).