The prevalence of simian lines in 1,347 children attending a Turkish pediatrics outpatient polyclinic was found to be 3.34 per cent. There were twice as many cases of unilateral simian lines (2.23 per cent) as bilateral (1.11 per cent). The difference between the frequency of simian lines in males (4.52 per cent) and females (1.64 per cent) was statistically highly significant (P < 0.001). It is suggested that one of the genetic factors determining the presence of simian lines may be partially sex-linked. The prevalence of simian lines in this survey is discussed in relation to similar observations in other countries, and tends to support the suggestion that there is an increasing prevalence from western to eastern countries. No relationship was found between blood groups and simian lines.