This is a preliminary report on an aspect of single-sex education in British secondary schools. It is based on a reanalysis of data on students' performance in mathematics in the General Certificate of Secondary Education (GCSE) in 2001. Students' attitudes to mathematics were also investigated. Girls' achievement within girls' schools was slightly higher than for girls in mixed schools, after adjustment for a range of factors. On average, the difference was about one tenth of a standard deviation. Boys, on average, performed less well in boys' schools than in mixed schools by about the same amount. In terms of girls' attitudes to mathematics, attendance at a girls' school was associated with more positive attitudes, again by about a tenth of a standard deviation. Boys' attitudes to mathematics were not significantly different between those attending boys' schools and those in mixed schools, however.