Hurricane Agatha left at least 11 people dead and 33 missing in the southern Mexico state of Oaxaca, where the storm set off flooding and landslides, Governor Alejandro Murat said on Wednesday.
More than 40,000 people in the state have been affected, primarily along the coast and in the mountains beyond, Mr Murat said.
Agatha was the strongest hurricane since records have been kept to come ashore in May in the eastern Pacific.
It made landfall on Monday afternoon on a sparsely populated stretch of small beach towns and fishing villages as a strong Category 2 hurricane with maximum sustained winds of 170 kilometres per hour, but it quickly lost power moving inland over the mountainous interior.
Even as Oaxaca continued to search for the missing and clean up downed trees and flooded homes, Mexican officials were watching another large area of thunderstorms along the Caribbean coast of the Yucatan Peninsula that forecasters said could become a tropical storm later this week.
The US National Hurricane Centre gave the system a 70 per cent chance of becoming a tropical depression in the next 48 hours. As it took shape, it dumped heavy rain on southern Mexico and Belize.
Mr Murat said rivers overflowed their banks and swept away people in their homes, while other victims were buried under mud and rocks.
He added the deaths appeared to be concentrated in a number of small towns in the mountains near the coast but that there were also reports of three children missing near the resort of Huatulco.
He said power had been restored to some communities near the coast, but that some bridges had been washed out and mudslides blocked a number of motorways.
San Isidro del Palmar, only a few kilometres inland, was swamped by the Tonameca River that flows through town.
Residents swam through neck-deep water to salvage what items they could from their homes, moving carefully with piles of clothing on top of their heads and religious figures in their arms.