Introduction A survey to determine the prevalence of trachomatous trichiasis (Tl] requires a large sample size and the recommended participant age is 2': 15 years. This study sought to establish the appropriate age range of individuals to be included in Tl surveys. Methods Data from six previous surveys of adults 2': 15 years old were reanalysed. Results Reanalysis indicated that 69.6-93.3% (average 87.0%) of untreated n occurred in those aged 40+ years and 52.2-86.7% (average 73.1%) in those aged 50 + years (age 2':50 years is used in rapid assessment of avoidable blindness). Age 2':40 years was adopted in a Tl survey conducted in Turkana district because it allowed a smaller sample size than age 2':15 years. Conclusions The estimated backlog of untreated Tl in people aged 2':40 years old in Turkana was 5932 and the overall n backlog was likely to be 6358-8523. These findings cannot be generalised because all surveys were carried out in the same country.