Whole-genome sequencing (WGS) can provide valuable biological information about tumours and is increasingly being shown to have the potential to influence cancer treatment planning. National initiatives that include WGS for certain patients with cancer have been introduced in some countries, such as England, Australia, and Sweden. Unlike specific, validated genetic tests that are commonly used clinically to guide treatment decisions and are predictive of response, the effect of WGS on long-term patient outcomes has not yet been well studied and is broadly unknown.