Numerical beam propagation simulations are used to demonstrate that th
e distributed thermal lensing, produced by the absorption of the pump
laser in the gain medium, profoundly affects the operation of hard-ape
rtured, Kerr-lens modelocked Ti:sapphire lasers. The pump-induced ther
mal lensing is shown to shift and distort the resonator stability regi
ons (even allowing the regions to overlap) and severely perturb the mo
delocking mechanism in one of the stability regions.