Interest in the formulation of quantitative structure-activity relationships (QSAR) for antitumor drugs has begun to develop in the past decade. The work in this area has been reviewed briefly, using as examples studies on nitrosoureas, aniline mustards, and aryl triazenes. A salient conclusion from this analysis is that the present drugs in clinical use are more hydrophilic than one might expect. The reason for this may be that they have been developed using leukemia as the test system which may in part account for the fact that while the currently used drugs are effective against leukemia, they are not effective in general against solid tumors.