Synaesthesia is a condition in which a mixing of the senses occurs; for example, sounds trigger the experience of colour. Previous reports suggest this may be familial, but no systematic studies exist. In addition, there are no reliable prevalence or sex-ratio figures for the condition, which is essential for establishing if the reported sex ratio (female bias) is reliable, and if this implicates a sex-linked genetic mechanism. Two independent population studies were conducted in the city of Cambridge, England (studies 1 and 2 here), as necessary background to the family genetic study of synaesthesia (study 3). Studies 1 and 2 arrived at an almost identical prevalence rate for synaesthesia: approximately 1 case in 2000. The sex ratio found was 6:1 (female:male). A third of cases also reported familial aggregation. In study 3 six families were examined, and first-degree relatives were tested for genuineness of the condition. All six families were indeed multiplex for synaesthesia. Alternative modes of inheritance are discussed.