Objective To compare the morphology and size of stem cells from two mammals of noticeably different body size. Design Observational study. Setting The Netherlands. Participants A humpback whale (Megaptera novaeangliae) and a laboratory mouse (Mus musculus). Main outcome measures Morphology and size of mesenchymal stem cells from adipose tissue. Results Morphologically, mesenchymal stem cells of the mouse and whale are indistinguishable. The average diameter of 50 mesenchymal stem cells from the mouse was 28 (SD 0.86) mu m and 50 from the whale was 29 (SD 0.71) mu m. The difference in cell size between the species was not statistically significant. Although the difference in bodyweight between the species is close to two million-fold, the mesenchymal stem cells of each were of similar size. Conclusions The mesenchymal stem cells of whales and mice are alike, in both morphology and size.