Cyclic esotropia, alternatively called ‘day squint’, is an intermittent disorder. It usually occurs in repeated cycles of every 48 hr (Hutcheson & Lambert 1998). The fusion and binocularity are usually defective on the strabismic days; in contrast, it remains normal during the straight days. Many studies have tried to clarify the pathophysiology of this rare condition, but exact mechanisms remain unknown (Metz & Searl 1984). We present a monthly cycle of these squints with misalignment associated with the days of the menstrual period that has never been reported yet. A 46-year-old female was referred for ocular pain. She had already been diagnosed with orbital pseudotumor at another clinic and took oral steroids for this condition. She was noted to have 45PD esotropia with diplopia. Six months later, her esotropia had improved without diplopia. We could not see her again until 4 years later when she presented to our clinic for a history of a squint, which was worse during her menstrual period. It typically lasted for 2 weeks each month. At that time, her ocular examination showed orthotropia and no diplopia, and visual acuity was 20/20. Two weeks later, she was noticed to have 35PD esotropia, but no change of visual acuity or refraction error was seen (Fig. 1). Her ocular motility was normal, but binocularity did not evaluated. We suggested a self-diary should be started, so she recorded daily whether she had straight eyes or a manifest squint. The diary clearly showed that the manifest deviation did predictably present during her menstrual periods. The cycle started on the first day of menstrual period, and it terminated at the end of menstrual period. Cyclic fluctuation usually occurred over one-month time frame. For evaluating the recurrence of orbital pseudotumor, she received CT scans, but we did not find any abnormalities on the CT scans (Fig. 1). The cycle had lasted for 4 months, and the strabismic period became more prolonged as time went on, and at last, her esotropia has lasted for 1 month. Finally, the periodic squint became a constant 40PD esotropia. The patient underwent bilateral medial rectus muscle recession (right, 5.5 mm; left, 5.0 mm). Following the surgery, she has remained orthotropic and free of diplopia during her menstrual period. Cyclic esotropia can be idiopathic or consecutive and also occur with defects of the central nervous system (CNS) (Pillai & Dhand 1987). The proposed mechanism of cyclic esotropia is a disruption of the innate biological clock associated with the CNS (Hutcheson & Lambert 1998). Some authors have suggested that a lesion of the suprachiasmatic nucleus of the hypothalamus could disrupt the biological clock and lead to cyclic esotropia (Pillai & Dhand 1987). Our case supports this possible mechanism in that the suprachiasmatic nucleus plays an important role in the course of menstruation (Norman et al. 1976). Several authors proposed that patients with cyclic esotropia have a basically strabismic condition with appended ‘bursts of orthophoria’ (Hutcheson & Lambert 1998). After visual maturity, late-onset cyclic esotropia could develop from conditions that disrupt fusion or cause diplopia (Hutcheson & Lambert 1998). The trigger event could be ocular surgery or CNS problems (Pillai & Dhand 1987). In this case, we could propose that constant esotropia by orbital pseudotumor might disrupt fusion. It is hypothesized that underlying this strabismic condition, our patient had entire days except her menstrual period where she experienced an orthotropic burst; therefore, a trigger factor to suppress orthotropic burst condition might also exist. The trigger factor could be menstrual hormones or unknown endogenous CNS transmitters stimulated by menstrual hormones. In conclusion, we present the first unique case of periodic esotropia with a monthly cycle and onset along with the menstrual period. In our patient, the development of cyclic esotropia could be related to the disruption of an innate biological clock, and the cycle was probably the result of some interacting factors related to the suprachiasmatic nucleus of the hypothalamus.