To improve clinical interpretation and use of in vitro clonogenic assay results, the authors reviewed their experience to date with chemosensitivity testing of over 1500 solid tumors. All clonogenic assays were performed using a double-layer-soft-agar system with continuous exposure of cells to one concentration of standard anticancer drugs. Significant growth was defined as greater than or equal to 30 colonies/control plate. Clinical responses were determined according to standard criteria. Data were analyzed using two different criteria of in vitro sensitivity (greater than or equal to 50% and greater than or equal to 75% inhibition of colony formation) and independently for each histologic type of tumor. Overall, 68% of specimens plated produced significant growth in vitro. Cloning ability varied from 57% to 82% depending on tumor histology. The assay was 57% reliable for predicting in vivo sensitivity, and 92% reliable for in vivo resistance. Predictive accuracy for sensitivity varied from 30% to 86%, depending on the tumor histology. Use of greater than or equal to 50% ICF (inhibition of colony formation) as criteria for differentiating sensitivity from resistance proved most reliable, although criteria should be individualized for each tumor type to maximize predictive accuracy.