Experimental metastases are often located preferentially near the surfaces of liver and lungs. There are indications that some surface preference also exists in human metastasis. Since this phenomenon has thus far received little attention, we quantitatively analyzed the distribution of metastases in mouse liver and lungs, produced by intraportal or intravenous injection of B16 melanoma cells. We also compared the distribution of these metastases to that of initially arrested tumor cells. The main findings were: The metastases had a highly significant surface preference. Most liver metastases were roughly spherical, but those in lungs were usually exceedingly flattened, a shape apparently imposed by the structure of the invaded tissue. Initial tumor cell arrest occurred randomly in the two target organs, and is therefore not responsible for the irregular distribution of the metastases found later.