The number of suicides in English and Welsh prisons is increasing, but the excess compared with the general population has not been reliably quantified. We therefore compared, in narrow age bands, all 1312 suicides of male prisoners in England and Wales between 1978 and 2003 with suicide rates in the general male population. The overall standardised mortality ratio for suicide was 5.1 (95% CI, 4.8-5.3), suggesting a five-fold excess of suicides in male prisoners, with a particularly striking excess in boys aged 15-17 years (standardised mortality ratio 18 [13-26]). The proportional excess of suicides of male prisoners has been increasing during the past quarter of a century, which underscores the need for substantial improvements in suicide prevention in prisons.