Our objective was to delineate the relevance of the personality construct alexithymia and anger-in in patients with fibromyalgia syndrome. Fifty subjects with fibromyalgia syndrome were compared to 20 subjects with rheumatoid arthritis and 42 healthy controls on the measures of anxiety, depression, anger, alexithymia, pain intensity and disability. There was a significant difference on the measures of anxiety and anger between FMS and RA groups, and also between FMS patients and healthy controls. There was a significant difference between FMS patients and healthy controls on the measures of depression, difficulty in identifying feelings subscale of TAS (TAS-dif), and total alexithymia scores. When the severity of pain was controlled for, there was a significant difference on the measures of anger and alexithymia between the FMS and the RA groups. Fibromyalgia patients were more alexithymic than rheumatoid arthritis patients even when the level of depression was controlled for. Anger towards oneself, which is anger-in, was higher in patients with fibromyalgia patients than in the rheumatoid arthritis sample. A stepwise regression model showed that the anger-out scores and the anxiety scores predicted the level of pain severity, and this explained 32% of the variance in the fibromyalgia syndrome group. Although anger-in is consistently higher in fibromyalgia patients, it is the behavioral expression of anger, together with anxiety, that predicts the severity of the pain. The difficulty of identifying feelings, rather than other dimensions of alexithymia, seems to be associated with fibromyalgia.