QTLs with large effects on grain yield under drought stress have been detected in two rice populations.  In a cross between a highly-tolerant upland cultivar, Vandana, and a susceptible Indonesian cultivar, Way Rarem, yield under severe stress was evaluated in 458 random F_3:4 lines at IRRI in the 2005 and 2006 dry seasons. Severe upland stress was imposed by repeatedly withholding irrigation from maximum tillering until harvest.  In each stress cycle, irrigation was withheld until soil water tensions reached -60 kPa at 30 cm.  A major QTL affecting yield under stress was detected on chromosome 12 near SSR marker RM511, explaining 40% of the genetic variation. Homozygotes for the yield-enhancing allele, which came from the susceptible parent, Way Rarem, yielded over 200 kg ha^-1 more than the contrasting homozygote in stress trials in which the mean was less than 1 t ha^-1.  The Way Rarem allele also increased biomass production under vegetative-stage stress.  Effects of different alleles in this region on yield under stress have been detected in two other backgrounds.  A second large-effect QTL, near RM256 on chromosome 8, was detected for yield under upland drought stress at the reproductive stage in a population derived from a cross between IR64, a susceptible lowland variety, and IR55423-01, an improved upland rice variety.  In drought trials with mean yield of between 0.5 and 1 t ha^-1 in the dry season of 2003 and the wet season of 2004, the allele from IR55423-01 increased mean yield by at least 200 kg ha^-1, but had no effect on yield under transplanted lowland conditions. These are the first reports of QTLs with major effects on rice grain yield under drought stress evaluated in the field and repeated across seasons.  Both loci are being fine-mapped and will be assessed for their usefulness in marker-aided breeding.