Evening primrose oil, which includes GLA, is one of the most popular of many treatments available for PMS. It has been reported that diagnosis of PMS on the basis of prospective daily rating is essential. However, few studies on alleviation by GLA of the symptoms of PMS have been performed using prospective daily rating. We examined the alleviating effect of GLA on symptoms of PMS diagnosed by prospective daily rating. Twenty-eight women diagnosed with PMS consumed vegetable oil containing about 180 mg/day of GLA for three luteal phases in a randomized, double-blind, parallel protocol, and were examined for duration and severity of PMS symptoms. Levels of DGLA in plasma phospholipid were significantly lower in women with PMS than in those without it (DGLA, P < 0.01) before treatment regardless of phase in menstrual cycle. After GLA administration, the levels of GLA and DGLA in plasma phospholipid in the GLA group were significantly higher than those both in the placebo group and before treatment. Improvement of the duration and severity of PMS symptoms as a whole, as well as that of irritability, was significantly more pronounced in the GLA than in the placebo group. These findings indicate that GLA can be effective for treating the symptoms of PMS, and that GLA and DGLA in plasma phospholipid may play a role in the onset of PMS.