Binding energy of excitonic molecules in GaAs/Al{sub}0.3Ga{sub}0.7As quantum wells is calculated by the diffusion Monte Carlo method in the effective mass approximation. The binding energy increases with decreasing well width, d, and is 2.1 meV at d= 40 A. The ratio between the binding energy of excitonic molecules and that of excitons, is about 0.17 and slightly decreases as d increases. The calculated binding energy agrees well with experimental ones for d> 100 A, but is smaller than those for d< 100 A.
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