Recent concerns have been raised regarding whether antidepressants, especially selective serotonin reuptake inhibitors (SSRIs) might increase suicidal tendencies and intense debate-rages over the pros and cons of their use. Although systematic reviews and population-based studies have been conducted, a consensus on this association remains to be established. Subsequently, the concept of so-called 'activation syndrome' associated with antidepressants has been accepted without its adequate verification. In the present report, we present our experience of seven cases considered of having 'activation syndrome' brought on by antidepressants, and examine its clinical relevance to bipolar spectrum disorder (Ghaemi, et al., 2001) both symptomatologically and diagnostically. Five patients, diagnosed as having major depressive disorder according to the diagnostic manual (DSM-IV), met the criteria of bipolar spectrum disorder and suffered from activation syndrome following the administration of SSRIs, mainly paroxetine. Similarly, hypomania developed in all five cases with depression; the diagnostic criteria of a hypomanic episode were not met. In the remaining two patients, who were both diagnosed with bipolar disorder, one showed irritability and insomnia through imipramine use, and the another developed a hypomanic and/or a mixed state after the co-administration of fluvoxamine and trazodone. From the results of our examination, 'bipolarity', which is the pivotal factor of bipolar spectrum, might exist behind the phenomenon recognized as activation syndrome, and be revealed by antidepressant treatment, just like manic switching. Moreover, the various problems encountered in the current practice of treating mood disorders, including unipolar-bipolar dichotomy, manic switching by antidepressants, and narrow criteria for a mixed episode, were pointed out a new through this concept of activation syndrome. Actually, the understanding of activation syndrome clinically leads to the prevention of suicidal behavior and the careful use of antidepressants for bipolar (spectrum) disorder, but we must be prudent when applying this concept, since it has not yet been established.