Using a case registry, we investigated the temporal occurrence of moto
r neuron disease (MND) on Guam. MND with onset during 1941-85 was docu
mented in 434 Chamorros and 9 non-Chamorro migrants who had lived on G
uam before onset. Increased median age at onset and decreased age-adju
sted incidence rates (since the early 1960s) were observed for Chamorr
os of both sexes. Our evidence about MND on Guam is consistent with: 1
) The latent period duration has varied from years to decades; 2) With
time, the exposure period or latent period, or both, have lengthened;
3) The high risk of acquiring the condition has been reduced since, a
t least, the early 1950s, and the most recent years of meaningful risk
were the early to middle 1960s; 4) The critical age for acquiring the
condition is in adolescence and adulthood; 5) Change of environment f
rom Guam to overseas during childhood resulted in decreased risk of ac
quiring the condition.