The amount of glandular differentiation was quantitated in 64 cases of primary human mammary ductal carcinoma (PR) and their axillary lymph node metastases (LNMs) to identify differences between the two groups and their effect on patient prognosis. A significant difference between a PR and any of its LNMs was uncommon (less than 5% of the cases), and variation between each PR and its LNMs was directly proportional to the amount of differentiation within the PR itself. Variance among the LNMs was proportional to that found within the PR. Each LNM usually varied as much within itself as it did from the other LNMs. Prognosis was not affected by the LNMs being less differentiated than the PR, nor vice versa. This study has provided no evidence for selection of a morphologically more aggressive tumor clone during this phase of the metastatic process, but selection by other morphologic or functional parameters is not excluded.