It is often stated that the majority of large bowel neoplasms can be diagnosed by rigid sigmoidoscopy since they lie within range of the instrument (I). A prospective study was undertaken by one clinician to determine how often in practice it is possible to make a full sigmoidoscopic examination and to attempt to determine some factors that may be responsible for the failure to d o so.Rigid sigmoidoscopy was carried out in 238 consecutive new outpatients. The examination was performed without preparation of the bowel using Lloyd-Davies' pattern instrument (length 25cm, diameter 1.8cm) with the patient in the left lateral position. The furthest distance passed was recorded and, where the examination was considered to be limited, the reason was given to be the result of discomfort of the patient, the inability of the examiner, the presence of faeces obscuring vision or pathological narrowing of the bowel.The furthest distance the instrument could be passed was less than 14cm in 18 patients (7.5 per cent), from 14 to less than 16cm in 54 (22.7 per cent), from 16 to less than 22cm in 52 (21.9 per cent) and greater than 22cm in 114 (47.9 per cent). Examination beyond 16 cm was possible in 113 of 138 men (82 per cent) and 53 of 100 women (53 per cent), a significant difference (2' = 26.1; P