The Papanicolaou stain is a gold-standard staining method for tumor diagnosis in human cytology. However, it has not been used routinely in veterinary cytology, because of its complicated multistep procedure and requirement for wet fixation. Currently, a rapid Papanicolaou stain using air-dried smears is utilized in human cytology, but usefulness of this rapid-air-dry Papanicolaou (RAD-Pap) stain in the veterinary field has not been fully evaluated. The purpose of this study was to evaluate the usefulness of the RAD-Pap stain by using quantitative analysis. Air-dried impression smears were collected from tumor specimens and stained with RAD-Pap and Giemsa. Twelve parameters representing the criteria of malignancy were quantitated, and characteristics of the RAD-Pap were evaluated statistically. The RAD-Pap stain could be applied to all the smears, and images of nucleoli and chromatin patterns were clear and detailed. In quantitative analysis with the RAD-Pap stain, but not with the Giemsa stain, dispersion of nucleolus size and dispersion of nucleolus/nucleus ratio in malignant tumors were significantly higher than those in benign tumors. These findings demonstrated that the RAD-Pap stain was useful for obtaining detailed nuclear information, and the ability to differentiate benignity and malignancy by nucleolus findings was a principal advantage of this stain. This RAD-Pap stain could be routinely used as a supportive staining method in veterinary diagnostic cytology.