Late-onset circulatory collapse (LCC) in preterm infants is presumably caused by relative adrenal insufficiency. Because eosinophilia is known to be associated with adrenal insufficiency, we attempted to clarify the relation-ship between eosinophilia and LCC in preterm infants. We divided the cases of the infants (born at < 28 weeks' gestation) admitted to our neonatal intensive care unit in 2008-2010 into 2 groups: those diagnosed with LCC that received glucocorticoids (LCC group), and those who did not receive glucocorticoids (control group). We compared eosinophil counts between the 2 groups and between before and after glucocorticoid treatment in the LCC group. A total of 28 infants were examined: LCC group (n = 12); control group (n = 16). The peak eosin-ophil counts of the LCC group were significantly higher than those of the control group (median: 1.392 × 109/L vs. 1.033 × 109/L, respectively; p = 0.02). Additionally, in the LCC group, the eosinophil counts declined significantly after glucocorticoid treatment (0.877 × 109/L vs. 0.271 × 109/L, p = 0.003). Eosinophil counts in the LCC group were significantly higher than in the control group and decreased rapidly after gluco-corticoid treatment. These results indicate that eosinophilia may be a factor associated with LCC caused by adrenal insufficiency.