It is well known that the sacrococcygeal joint may be obliterated by ossification, producing fusion of the first coccygeal segment with the sacrum. There is a conspicuous lack of quantitative information on the occurrence of such bony fusion. It is generally regarded to be characteristic of old age, whilst conflicting statements surround the question of sex differences in its frequency. This report describes the occurrence of sacrococcygeal fusion in two adult British populations, one from Aberdeen and the other from London. In the Aberdeen group, both males and females had a similarly high incidence. In the London group, the males exhibited an intermediate rate whereas the females showed a relatively low occurrence. In London, the males showed a delayed onset of sacrococcygeal fusion with significantly fewer cases occurring below 40 years of age. In contrast, the London females showed a similar frequency of fusion below and above age 40. The effects of age could not be analysed in the Aberdeen group owing to the paucity of subjects below middle age. The findings of this investigation indicate that the occurrence of sacrococcygeal fusion is not related exclusively to age and sex. It is postulated that other factors of a genetic and/or environmental nature are involved.