Capture-recapture analysis allows an investigator to estimate the number of unobserved cases in an epidemiological survey, although it has not been employed widely in published research to date. This study examines the usefulness of capture-recapture analysis in measuring the incidence and prevalence of motor neuron disease (MND) in Northern Ireland (NI). Cases were ascertained from the NI MND register to measure incidence and prevalence. The authors then employed capture-recapture analysis to estimate the number of missing cases. Less than one case was estimated to be missing from the prevalence study, providing a completeness of ascertainment of 99%. Prevalence was 3.3 per 100,000 population adjusted to the European standard population and was unchanged when adjusted for missing cases. Incidence was 1.4 per 100,000 person-years adjusted to the European standard population. Due to the lack of overlapping sources it was inferred that there were no missing cases and completeness of ascertainment was estimated at 100%. In conclusion, capture-recapture analysis is relatively simple to perform and provides an objective measure of case ascertainment in epidemiological studies. The authors feel that future investigators should consider employing capture-recapture methods, where possible, to validate their estimations of disease frequency in MND.