From 1978 through 1995, a sex ratio of 6.6:1 of boys to girls (N = 275) was observed for children referred to a specificity clinic for gender identity disorder. This article attempts to evaluate several hypotheses regarding the marked sex disparity in referral rates. The sexes did not differ on four demographic variables (age at referral, IQ, and parent's social class and marital status) and on five indices of general behavior problems on the Child Behavior Checklist; in addition, there was only equivocal evidence that boys with gender identity disorder had significantly poorer peer relations than girls with gender identity disorder. Although the percentage of boys and girls who met the complete DSM-III-R criteria for gender identity disorder was comparable, other measures of sex-typed behavior showed that the girls had more extreme cross-gender behavior than the boys. Coupled with external evidence that cross-gender behavior is less tolerated in boys than in girls by both peers and adults, it is concluded that social factors partly account for the sex difference in referral rates. Girls appear to require a higher threshold than boys for cross-gender behavior before they are referred for clinical assessment.