Scott McKenzie, who performed the 1967 ballad “San Francisco (Be Sure to Wear Flowers in Your Hair),” which became a defining hit for the counterculture generation and helped draw tens of thousands to the Haight-Ashbury district for the Summer of Love, died on Saturday at his home in Los Angeles. He was 73.

The cause was unknown, said Dr. Frank Snyder, one of his physicians. A Web site devoted to Mr. McKenzie said that he had been ill for several weeks and that he suffered from Guillain-Barré syndrome, a disorder that causes the immune system to attack the nervous system.

“San Francisco” was written by John Phillips, a founder of the Mamas and the Papas, who had been a friend of Mr. McKenzie’s since high school. The two started a band called the Journeymen, which recorded several albums in the 1960s.

In the song, Mr. McKenzie sang lyrics like these with a slow, almost mournful cadence:

All across the nation, such a strange vibration.

People in motion.

There’s a whole generation, with a new explanation.

“San Francisco” hit a nerve with people looking to protest what they saw as an unjust social order, and it rocketed to No. 4 on the pop charts.