Experiments have been carried out to study the ionization buildup and gas breakdown processes in the focal volume of an intense 10. 6‐&mgr; laser pulse in atmospheric‐pressure helium preionized to various levels of electron density. With no preionization the laser flux required for breakdown was in agreement with results obtained by previous authors, and showed the same dependence on the focal volume size. However, with a high initial electron density, the threshold was much lower and showed no volume dependence. The results indicate that with preionization the breakdown threshold is controlled by cascade ionization processes, while with no preionization it is dominated by other effects such as small impurity particles in the gas, and that these other effects may be responsible for the diameter dependence of the breakdown threshold.