We report the first results on the influence of ambient pressure on single bubble sonoluminescence.The equilibrium radius as well as the maximum radius of the bubble increased as the ambient pressureover the liquid head was decreased. The number of photons emitted during a sonoluminescence pulseincreased linearly with a decrease of ambient pressure. These results are in good agreement with thetheory of the ambient pressure effect [L. Kondic et al., Phys. Rev. E 57, R32 (1998)] and with therecent dissociation hypothesis.