In this study, the pitfalls of tumor measurement in the nude mouse were evaluated. Regarding intermethod variation, diameters of subcutaneous tumors in nude mice were expressed as length, area, and volume; tumor weights were also recorded. These measurements were all compared to a reference standard: water displacement volume. Estimates of area and volume derived from caliper measurements correlated well with water displacement volume (r = 0.97 and 0.98, respectively). At necropsy, tumor weight was the most consistent and reproducible reflection of tumor volume (r = 1.0000). Regarding interobserver variation, mean absolute difference among volumes determined by several investigators who measured the same tumors in living animals was determined. This averaged 15% of the mean calculated volume. Regarding intraobserver variation, observers measured four separate masses in nude mice eight times each. The observers were prevented from realizing that the same animals were being repeatedly evaluated. Volumes were compared in order to quantify the average variation that occurs when a single investigator repeatedly measures the same mass. When large masses were measured, this error was 7%; when small masses were measured, the error was 27%. Recommendations are made for future work employing tumor measurement.