There is a well documented predominance of males diagnosed with neurodevelopmental disorders. The influence of sex steroids upon brain development has been suggested to mediate sex differences in developmental psychopathology, and has been epitomized in the 'extreme male brain theory'. The objective of this study was to extend previous studies dealing with the extreme male brain theory and to study the sex ratio (proportion of males) in the siblings of 469 individuals with a developmental language disorder (DLD) who were consecutively assessed in the same clinic during a period of 10 years. Among their 908 live-born siblings, 503 were males and 405 females. This yields a sex ratio of 0.554, which is significantly higher than the Danish live birth sex ratio of 0.514 over the same period (P = 0.02). Our findings are consistent with the hypothesis that male sex hormones may be implicated in the aetiology of DLD.