Mortality statistics in amyotrophic lateral sclerosis (ALS), which is more commonly and generally termed motoneurone disease (MND) in the United Kingdom, have been shown to reflect the incidence of previously diagnosed cases of the disease in a more complete way than in other conditions [1,2]. An analysis of changing patterns of mortality may therefore be a particularly appropriate way of tracing the underlying trends in the disease and is in principle a useful way of investigating the relationship between environmental and genetically controlled factors in the genesis of the condition. The majority of analyses so far have concentrated on the crude rise in reported mortality rates evident in recent decades in a number of countries [2-5], on the uneven geographical distribution [6,7] and on the complex range of plausible causes for these reported rises. Debates have centred on whether the increases represent 'real' or 'artifactual' changes, with no apparent resolution of the issue [8,3]. Recently Riggs [9] proposed a novel way of analysing this issue by using a Gompertzian model and provided evidence of the existence of an inherently susceptible subset of the US population. Riggs indicated that while the rise in ALS mortality is real, it is for the most part the result of an increase in the size of this inherently susceptible sub-population due to greater longevity. In order to examine the wider applicability of a Gompertzian model to ALS the technique has been replicated with the mortality rates for England and Wales for the 27-year period from 1963 to 1989. The technique has been developed and extended to produce an estimate of the size of the inherently susceptible sub-population (both male and female) over the entire period.