The energetics of the ring, bowl, and cage isomers of C-20 were determined in all-electron fixed-node quantum Monte Carlo calculations. In good agreement with results from valence-only quantum Monte Carlo calculations and from Moller-Plesset calculations with large basis sets, the calculations predict the lowest-energy isomer to be the bowl isomer. The energies of the ring and cage, relative to the bowl, are 1.1 +/- 0.5 and 2.1 +/- 0.5 eV, respectively.