We tested the ability of the in vitro humman tumor clonogenic assay (HTCA) to predict clinical response for patients with salid tumors. Patients had objectively measurable disease and received three or more courses of single or combination chemotherapy directed by HTCA results. The correlation between clinical responses and in vitro sensitivity was evaluated in 57 patients. Tumor types included breast cancer (25 cases), colon cancer (23 cases) and primary unknown adenocarcinoma (9 cases). In these 57 comparisons, overall predictive accuracy of HTCA for clinical sensitivity was 93%, predictive accuracy for resistance was 97%. Our results suggest that in vitro selection of effective drugs may avoid unnecessary toxicity while assuring therapeutic effects in palliative and adjuvant chemotherapy for the treatment of colon, breast and primary unknown adenocarcinoma.