In the study of cancer as a complicated disease caused by many combinations of various factors, it may be of importance to consider also any possible changes of the water structure in the environment of the malignant cells. The occurrence of such changes has been established experimentally, for example, by a study of the magnetic properties of water by nuclear magnetic resonance (NMR) spectra. It has been found that the protons in the water surrounding malignant cells have a much longer spin-lattice relaxation time than the protons in the water around normal cells. This indicates that the water molecules in tumor cells are less structured and able to move more freely than in normal tissues, where, due to the effect of hydrogen bonding, water occurs mainly as five- or six-membered rings. This prolongation of the proton spin-lattice time may be an important factor in cancer, but further studies are necessary before one can decide with certainty whether it would be possible to use this effect to diagnose malignant transformations at an early stage. It is suggested that changes in the magnetic properties of water in a malignant tumor during chemotherapy and other treatments be monitored as control tools.