Out of 348 child injured cyclists presenting at hospital emergency departments in Kingston only one injury (0.03%) - concussion - was serious enough to be admitted to hospital.
Source: Bicycle-related Injuries in Children: A Study in Two Ontario Emergency Departments, 1994, Hardy J Schwartz and Robert J Brison, Chronic Diseases Canada Volume 17, No.2 -1997

Out of 873 Ontario traffic fatalities in 2002, 20 were cyclists.
Source: Ontario Road Safety Annual Report 2002

There were 112 cyclists hospitalized in Ontario (1999/2000) for major injury. A portion of these would be for head trauma.
Source: The Ontario Brain Injury Association

There were 2494 patients with major head trauma (ISS >12) admitted to Ontario hospitals in 2001/2, each with at least one head injury. The average length of stay was 8 days per patient. Patients suffered a total of 4066 head injuries (average 1.63 per patient).

Injury causes were:
Motor Vehicle Traffic-related: 1896 (47%)
Falls: 1396 (35%)
Homicide: 274 (6.7%)
Motor Vehicle non-Traffic: 112 (2.75%)
Suicide: 75 (1.8%)
Cyclists: 54 (1.3%)*
Other 259: (6.4%)

* data breakout by number of victims is not made available; assuming cyclists suffered the average number of injuries per patient (1.63) about 33 of almost 2,500 head injured victims were cyclists.
Source: Ontario Trauma Registry Canadian Institute for Health Information

Editorial Comment

The data above are important in they relate only to injuries which could have disabling effects. Helmet law proponents who quote statistics in support of their position rarely distinguish between potentially disabling head injuries and vastly greater numbers of ones of a lesser severity (where recovery is assured). The latter are not pertinent to the helmet law debate in that if other injuries of lesser severity are to be used then the discussion would have to also include preventing injuries to other body parts using safety equipment like wrist guards, elbow pads and goggles.

Seriously head-injured cyclists are a tiny minority - little more than 1% of all seriously head-injured victims. Motor vehicle users together with pedestrians make up almost half of the total. It begs the question, "who should really be wearing the helmets?" Maybe helmet proponents who believe helmets protect against fatal and disabling injuries would answer this.