Individuals with 46, XY pure gonadal dysgenesis present with a completely female phenotype. These individuals develop bilateral streak gonads and have normal Müllerian structures. The apparent absence of testicular tissue in these individuals suggests a mutation in the initial steps of the male sex-determining pathway. A candidate gene for the primary signal in this pathway was recently cloned (SRY) which encodes a protein with a DNA-binding capacity. In a study of 14 XY females with pure gonadal dysgenesis harbouring SRY, we analysed the histology of the gonads and compared it to the presence or absence of mutations in the SRY open reading frame (SRY-orf). The histological analysis revealed two distinct groups of streak gonads. In the first group, the gonad was composed of exclusively ovarian-like stroma, with sclero-hyaline nodules in some areas. No tubules were observed. The gonads in the second group were composed of undifferentiated stroma harbouring either tubules or a rete structure. This suggests that in the latter group some differentiation (towards testis formation) has occurred, whereas in the first group ovarian differentiation has been interrupted. Individuals with mutations in the SRY-orf were found to have streak gonads of the first group, whereas most of the remaining XY females without detectable mutation in the SRY-orf had streak gonads belonging to the second group. On the basis of histology, it may be possible to distinguish between mutations in the sex-determining or sex-differentiation pathways. We suggest that SRY may play a role in rete testis formation.(ABSTRACT TRUNCATED AT 250 WORDS)