A theoretical model for the thermal lens and birefringent effects induced in a continuously pumped Nd:YAG rod is developed. Experimental measurements of these effects using a 6328‐Å probe are described and compared to theory. Excellent agreement between the two types of measurements and theory is obtained. The thermally birefringent and bifocusing phenomena are suggested as causes of reductions in the polarized and fundamentalTEM00powers from a high‐power multimode Nd:YAG laser. Typical results are described using a laser which operates in the 200−300‐W range of average powers.