Kemp's ridley (Lepidochelys kempii) is the most endangered of the sea turtles. Its female population in the Gulf of Mexico suffered a major setback sometime between the ends of nesting seasons in 2009 and 2010. Prior to that, annual nests (i.e., clutches laid by multiple year-classes of nesters) at the female population's index beach in Tamaulipas, Mexico were increasing exponentially, the result of more than 4 decades of cumulative conservation efforts on land and at sea. Annual nests dropped 35.4% in 2010 and remained well below predicted levels through 2014, and annual hatchlings released (both sexes combined) also were lower in 2010–2014 compared with those in 2009. We conducted novel analyses of an available 1966–2014 time series of annual nests and annual hatchlings released on the index beach. We examined 1) the relationship between time-lagged annual nests during years 1986–2014 and cumulative hatchlings released by years 1976–2004, respectively, assuming female minimum age at maturity of 10 yrs, and 2) the time-series of time-lagged annual nests during 1986–2014 divided by cumulative hatchlings released by 1976–2004, respectively, under the same assumption. Both metrics showed extraordinary downward departures in 2010–2014, instead of expected increases. Although causes of the population's setback have not been determined with certainty, we suggest that the most expedient way to restore this population's growth would be to translocate more clutches to protective corrals, leaving fewer in situ where their survival is reduced. It could take at least 10 yrs before results of such a change in conservation practice become evident.