BACKGROUND: Studies from around the world have shown that suicide risk is increased in cancer patients, but no previous detailed analysis has been carried out in England. METHODS: We calculated standardised mortality ratios (SMRs) for suicide in 206,129 men and 211,443 women diagnosed with cancer in South East England between 1996 and 2005, relative to suicide rates in the general population. RESULTS: We found a significantly increased risk of suicide in men (SMR 1.45, 95% confidence interval (CI) 1.20-1.73) and a moderately increased risk in women (SMR 1.19, 95% CI 0.88-1.57). In both sexes, relative risk of suicide was greatest in the first year after cancer diagnosis (SMR for men 2.42, 95% CI 1.84-3.13; SMR for women 1.44, 95% CI 0.82-2.33), and was also greater in individuals diagnosed with types of cancer with high fatality (SMR for men 2.67, 95% CI 1.71-3.97; SMR for women 2.17, 95% CI 0.80-4.73). CONCLUSION: There is a critical period immediately after the diagnosis of cancer during which the excess risk of suicide is particularly high. Carers need to be aware of the importance of attending to both the physical and emotional needs of cancer patients and cancer survivors.