A theoretical study is reported of the transition between the ground state ((1)A(g)) and the lowest triplet state (1 B-3(1u)) of ethylene based on the diffusion Monte Carlo (DMC) variant of the quantum Monte Carlo method. Using DMC trial functions constructed from Hartree-Fock calculations, complete active-space self-consistent field and multiconfiguration self-consistent field wave functions, we have computed the atomization energy and heat of formation of both states and the adiabatic and vertical energy differences between these states using both all-electron and effective core potential DMC methods. The ground-state atomization energy and heat of formation are found to agree with experiment to within the error bounds of the computation and experiment. Predictions by the DMC method of the triplet-state atomization energy and heat of formation are presented. The adiabatic singlet-triplet energy difference is found to differ by 5 kcal/mol from the value obtained in a recent photodissociation experiment. (C) 2003 American Institute of Physics.