Previous research suggested that sleep onset insomnia was significantly reduced with the use of relaxation techniques; however, the majority of these studies used college student populations with mild to moderate insomnia. The objective of the present study was to assess the effectiveness of using muscle relaxation training in a clinical population known to have sleeping difficulties—cancer patients. Results of this study suggest that muscle relaxation training is an effective technique to reduce sleep onset insomnia in cancer patients, and perhaps also in any clinical group. The technique seems especially promising since it was shown to be effective with severe insomniacs suffering severe medical problems. Results of the study were discussed in terms of possible explanations for the efficacy of the treatment, potential uses of the technique with other clinical populations, and ease of teaching nonpsychologist health professionals to treat with muscle relaxation training.