The time when the insult/triggering event occurs in Legg-Calvé-Perthes' (LCPD) is unknown. the purpose of this study was to determine, using the mathematical tool of incubation period modeling, the time of such event and the incubation period for LCPD. We reviewed 2,911 children with LCPD from 10 different centers around the world. They were divided into two groups: those from India (505 children, mean age 8.1 ± 2.3 years) and those from other than India (2,406 children, mean age 5.8 ± 2.2 years). A simple distribution with an excellent fit to the data was ln(y) = a + bx + cxln(x), where y is the proportion of children with LCPD at age of diagnosis x (r(2) = 0.994 for non-Indian and 0.959 for Indian children). The age of the triggering event was 1.32 years for non-Indian and 2.77 years for Indian children; the median incubation period was 4.30 years non-Indian and 5.33 years for Indian patients. Knowing the incubation period and age of triggering event narrows the number of potential etiologies in LCPD. this study does not support a prenatal triggering event as postulated in the past. similar incubation periods with different ages at diagnosis supports a common insult which occurs at different ages in different populations dependent upon local factors such as geographic location and ethnicity.