The advent of presymptomatic and prenatal testing in Huntington disease (HD) may change the reproductive behavior of persons at risk for HD. In order to assess future change, an analysis of fertility and reproductive fitness was carried out on 999 affected and 2,253 unaffected offspring from 235 New England families. Ascertainment biases observed for persons born before 1910 and after 1929 reduced the sample to 250 HD cases and 201 unaffected sib controls born between 1910 and 1929. No increase in reproductive rate was found in HD-affected men compared to male control sibs. A small increase in fertility averaging 0.5 child was seen in HD-affected females compared to unaffected females, but this difference was not significant. The increase in mean number of children for HD females is accounted for in part by a small number of affected women who had very large families. No evidence was found to suggest that any increase in reproductive rate for affected persons was related to offspring being born after HD onset. The fitness of both HD-affected and unaffected females was not significantly different from that of the general population of Massachusetts.