A treatment outcome study was conducted comparing a low-fat diet intervention with an education-support group and a waiting-list control group in the treatment of premenstrual tension syndrome (PMS) or Late Luteal Phase Dysphoric Disorder (LLPDD). Subjects met provisional diagnostic criteria for LLPDD and symptoms were monitored prospectively. A low-fat diet was hypothesized too be an effective intervention for reducing the severity of both physical and emotional symptoms in women suffering from LLPDD. This was based on the theory relating raised estrogen levels to premenstrual distress, and research suggesting low-fat diets reduce estrogen levels. The hypothesis that a low-fat diet would decrease premenstrual suffering was not supported by the results of this study. However, there appeared to be an advantage to participating in a group which provided support and information on LLPDD compared to receiving no treatment. Implications for future research, treatment recommendations, and methodological issues are discussed. / Ph. D.