We reviewed 22 post-laser (Nd:YAG laser) coagulation bladder washes collected immediately after treatment. All washes demonstrated a striking artifact of cellular spindling. These spindled cells occurred singly, in loose clusters, and in lamellar stacks and had elongated nuclei with dense chromatin and bipolar cytoplasm that was fused in the stacks. Concurrent biopsies demonstrated similar cytologic changes. The spindling is a nonspecific epithelial response to heat. Conventionally electrocauterized epithelia show this artifact in biopsies, but since only the base of the lesion and surrounding urothelium are subjected to heat with electrocautery, the relatively few spindled epithelial cells created presumably go undetected in cytology specimens. With laser treatment, however, the whole urothelial surface of the lesion is coagulated, producing a much greater number of spindled cells. It is important to avoid misinterpreting the spindled cells as cells from a mesenchymal neoplasm or a sarcomatoid carcinoma, mistakes that were made in some of our initial cases. Malignancy cannot be evaluated when cells exhibit spindling artifact; this judgement should be made on undistorted cells. Thus, pre-laser and post-laser washes should be submitted for evaluation of malignancy.