Large swarms of bees from several hives attacked the novice monks who were sweeping the grounds of the Luang Worawiham temple in Chiang Mai on Sunday.
Phra Ratcha Jetiyajarn, the temple abbot, said 76 novice monks were stung and rushed to three hospitals in central Chiang Mai.
Nineteen of them were in serious condition, said hospital director Dr Naren Chotirosnimitr.
Dr Naren said of those in serious condition, six arrived at the hospital in a coma. Their blood pressure had dropped dangerously low.
The director said 34 monks treated for minor injuries, adding bee attacks could be fatal if patients sustain multiple stings and are allergic to them.
Patients typically experience nausea and difficulty in breathing and develop a rash. In serious cases, their blood pressure drops sharply, Bangkok Post said.
Phra Ratcha Jetiyajarn said he had no idea what provoked the bees to attack.
The monks were carrying out their routine clean-up of the temple ground and had no problem with the bees before.
Despite the attack, the temple will keep the bee hives and will warn outsiders and tourists visiting the temple to stay well away from them, he was quoted as saying.