But looking back, the 56-year-old Auckland businessman reckons it was the best thing that could have happened to him.
Mr McCook said he was biking on the Clutha Gold Cycle Trail near Lawrence when he crashed last October.
He was taken to Dunedin Hospital and as part of his treatment, he had an MRI scan which showed he had a 1.2kg tumour on one of his kidneys.
"It was huge," Mr McCook said.
"When the doctor came in to see me, he sat on his haunches and said, ‘you’ve broken rib No6 on the right-hand side - and you’ve got kidney cancer’.
"I was shocked. I couldn’t believe it.
"Thank God the doctor did an MRI."
Statistics show kidney cancer kills more than 260 New Zealanders every year.
Fortunately, his tumour was discovered before it had metastasized.
Surgeons were able to remove it about a month later, and no chemotherapy or radiation therapy was needed.
Mr McCook said he had had previous crashes on his mountain bike - including one which had given him a serious head injury - but this would be the most memorable.
"I had no idea I had cancer. I had no symptoms - nothing.
"It could have killed me.
"That bike ride literally saved my life."