Breakthrough cancer pain (BTcP) is common, resulting in significant physical and psychosocial morbidity. We assessed the impact of BTcP on 81 cancer patients attending Irish specialist palliative care services. BTcP occurred up to twice daily in 24 (30%) and 3-4 times daily in 57 (70%) of cases. Median scores for the 'worst' and 'least' pains in the previous 24 hours were 7 and 2/10 respectively. Pain lasted < 15 minutes in 19 (23.5%), 15-30 minutes in 25 (30.8%), 30-60 minutes in 18 (22.2%) and > 60 minutes in 19 (23.5%) of patients. BTcP had a negative impact on general activity, mood, walking ability, work, relations with others, sleep and overall enjoyment of life. BTcP increased anxiety, depression, anger, isolation, financial difficulties and an inability to undergo cancer treatments. Systematic assessment of BTcP should form an integral part of every oncology/palliative medicine assessment. Once identified, BTcP should be managed assiduously.