In this paper we report the findings and data on a cross-sectional study of 50 pre-adolescent and adolescent girls with Turner syndrome. We confirm the presence of a typical cognitive profile in the different age groups with normal verbal intelligence contrasting with lower results on performal IQ subtests, related to relative weaknesses on visuospatial subtests i.e. "Block Design" and "Object Assembly". 5% of the girls with a "classical" Turner syndrome karyotype (i.e. 2/40) were mentally retarded versus 30% (i.e. 3/10) in the group with "rare" karyotypic anomalies. We noted a positive influence of hormonal therapy on the visuospatial functioning. No evidence was found for a high risk for behavioural problems. Hyperactive behaviour was seen in the youngest patients contrasting with a tendency to hypoactivity around the age of normal puberty. Problems in social development were noted from the age of primary school on resulting in social immaturity and even isolation. A proposal for guidance of Turner girls during the different developmental periods is given.