This commentary relates to the paper: J H Rees, R D Thompson, N C Smeeton, R A C Hughes. Epidemiological study of Guillain-Barré syndrome in south east England. This volume pp 74–7.

Ecologists have used capture-recapture methods for carrying out censuses of wildlife populations for many years. But only recently have epidemiologists realised that they can be applied to surveys of prevalence and incidence of disease. The underlying idea is fairly simple. The size of a population can be estimated by catching and marking a sample, then releasing that sample and allowing it to mix with the other members of that population before catching a second sample. The proportion of marked to unmarked subjects in the second catch will be the same as the proportion of the number in the first catch to the number in the whole population. The figure makes the principle clearer.