A general approximate criterion is developed (via WKB analysis) for the absolute instability of two parametrically pumped decay waves with one‐dimensional inhomogeneity. The absolute instability threshold pump power proves to be higher than that required for amplitude amplificationRby a factor &pgr;k’l2/27R, whenk’is the phase mismatchd&Sgr;ki/dxat the location of maximum instability andl−2is the effective curvature of the necessarily second‐order expansion about that point. Previous results are included as special cases.