Clinical findings are presented on 113 fragile X female heterozygotes from 44 families, based on physical examination, behavioural assessment, and reproductive history. In 85% of a subsample of 92 adult females non-verbal IQ score derived from the Block Design test was 85 or less. Verbal ability deficits were much less common. Typical facial characteristics, irregular teeth, and hypermobility of finger joints occurred in approximately 40% of adult females, but facial abnormalities were less common in children. Some physical anomalies and the level of intellectual impairment were, in adult carriers, associated with the presence of fragile X sites. The commoner physical anomalies or typical facial characteristics and intellectual abilities differed significantly between the known female heterozygotes and their 40 presumed normal relatives. Frequency of miscarriages was increased in fragile X females; in spite of this, a moderate increase in the number of children has been encountered in female carriers with borderline intellectual impairment. This important problem has genetical implications and needs further investigation. The importance is emphasised of a more detailed clinical examination of the females at risk in fragile X families.