OBJECTIVE: To show that the obstructive sleep apnea syndrome (OSAS) was the probable cause of D. Pedro II's excessive daytime sleepiness. METHOD: Research of historical documents and bibliographical. RESULTS: The excessive daytime sleepiness of D. Pedro II (1825-1891) was well known and bitterly criticized behavior by oppositionist magazines; it was also recognized by his peers. He would fall asleep in public places such as the theater and while attending lectures. As a youth, he was of normal complexion, putting on weight (obesity) at middle years. CONCLUSION: The possibility of this diagnosis is particularly relevant in this case because it points to an organic cause for D. Pedro II daytime naps and excessive daytime sleepiness. It could be the result of OSAS and not "disinterest" as erroneously assumed at that time.