Canine cutaneous mast cell tumors (ccMCTs) are currently graded according to Patnaik and Kiupel grading schemes. The qualitative and semiquantitative parameters applied in these schemes may lead to inter- and intraobserver variability. This study investigates the prognostic value of volume-weighted mean nuclear volume ([Formula: see text]), a stereological estimation that provides information about nuclear size and its variability. [Formula: see text] of 55 ccMCTs was estimated using the “point-sampled intercept” method and compared with histological grade and clinical outcome. The clinical history of dogs treated with surgical excision alone was available for 30 ccMCTs. Statistical differences in [Formula: see text] were found between grade II ([Formula: see text]= 115 ± 29 µm³) and grade III ccMCTs ([Formula: see text]= 197 ± 63 µm³), as well as between low-grade ([Formula: see text]= 113 ± 28 µm³) and high-grade ccMCTs ([Formula: see text]= 184 ± 63 µm³). An optimal cutoff value of [Formula: see text] ≥ 150 µm³ and [Formula: see text] ≥ 140 µm³ was determined for grade III and high-grade ccMCTs, respectively. In terms of prognosis, [Formula: see text] was not able to predict the clinical outcome in 42% of the cases; however, cases with [Formula: see text]<125 µm³ had a favorable outcome. These results indicate that, despite having limited prognostic value when used as a solitary parameter, [Formula: see text] is highly reproducible and is associated with histological grade as well as with benign behavior.