The tendency for the onset of a genetic disease to occur at progressively e
arlier ages or with progressively greater severity in successive generation
s is known as anticipation. Following the discovery of trinucleotide repeat
expansion as a plausible genetic mechanism for anticipation, interest in t
esting for anticipation has increased. Studies of anticipation can be biase
d when parents with late onset or children with early onset are preferentia
lly ascertained. This paper presents a nonparametric approach to testing fo
r age-at-onset anticipation that adjusts fur such preferential ascertainmen
t. The approach is illustrated through application to data on panic disorde
r.