Eighty-seven locoregional lymph node metastatic specimens from 52 gastric cancers and 35 colorectal cancers were tested using a human tumor clonogenic assay and the results were analyzed. Experiments were considered evaluable in 31 gastric and 21 colorectal specimens, respectively. Metastatic malignant cells and clonogenic cells per gram of wet specimen were significantly more prevalent in smaller (1.5 g weight or less) involved nodes (p < 0.005 and p < 0.025, respectively). The clonogenicity was also higher in the smaller (2.0 g or less) specimens, although the difference was not significant. In vitro chemosensitivity was higher in the smaller nodes and this tendency corresponded well to the clonogenicity. The data showed that the clonogenicity and in vitro chemosensitivity were high in the early phase of locoregional lymph node metastasis, and suggested that perioperative chemotherapy could control lymph node micrometastases and prevent recurrence in patients with gastric or colorectal cancer.