To assess completeness of case ascertainment by the Scottish Cancer Registry for the year 1992, we assembled a collection of databases containing potential registrations (excluding non-melanoma skin tumours and non-invasive neoplasms) from 14 separate sources relating to the population covered by one of the five regional registries. Apparently missed registrations were identified by linkage of these databases to cancer registration records. Their eligibility for registration was determined by reference in medical records, or when these were unavailable, by reference to the local Community Health Index (to establish residency at the time of diagnosis) in conjunction with the text of the original pathology report and/or the original death certificate. Misclassifications of site or incidence year were not regarded as missed cases. Of 517 apparently missed cancer registrations, 66 cases (3.5% of the revised total number of registrations of malignant neoplasms other than non-melanoma skin tumours for the study area in 1992) should have been registered as new independent primary malignant neoplasms, giving an overall estimate of completeness of 96.5%. The fact that so many apparently missed registrations were not eligible for registration illustrates the limitations of passive registration. Ascertainment of cases by the Scottish Cancer Registry appears to be high for most sites and compares favourably to the figures reported by registries outside Scotland.