Japan's royal palace say Princess Aiko, the only child of Crown Prince Naruhito and Crown Princess Masako, had been hospitalised because of a persistent high temperature.
The nine-year-old princess has been running a fever since the weekend, the imperial household agency said.
She "was hospitalised Tuesday for examinations and treatment... as the fever and symptoms such as a cough are continuing," a palace spokeswoman said.
The Japanese public reacted with horror in March last year when the palace announced Aiko, a grandchild of the emperor, had stayed away from school after suffering anxiety over "the wild behaviour of a few boys".
She later went back to classes accompanied by her mother, Crown Princess Masako, a former career diplomat who has mostly skipped her official duties through stress-induced mental illness over nearly eight years.
Aiko's birth in 2001 after seven years of marriage between Naruhito and Masako sparked heated debate on a possible change to laws of succession in Japan, which allow only males to inherit the chrysanthemum throne.
The crisis was resolved in September 2006 when Princess Kiko, the wife of Naruhito's younger brother, Akishino, gave birth to the imperial family's first male heir in four decades.
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