An index of condition was developed for wild Red Grouse chicks aged 10-30 days. This was derived from the difference between the observed weight and that expected from the chick's age, or stage of feather development. To measure it, the chicks needed to be caught only once. Variations in condition among different broods of wild chicks were greater than variations among broods of captive chicks in the same years, probably because the wild chicks' environment was harsher and more variable. Within years, the condition of wild chicks was not related to their brood size. Among years, mean condition was correlated both with breeding success and with the biomass of green heather food available in spring. This was because the chicks' condition and survival were poor for two years following an episode of severe weather damage to the heather.