To identify factors associated with participation in the 1986 Ontario, Canada doctors' strike, we surveyed 1,028 physicians; 69 percent responded, of whom 42 percent participated in the strike. Risk factors for participation included income greater than $135,000, being a surgeon or gynecologist, having previously "opted out" of the Ontario Health Insurance Plan, being professionally dissatisfied, being politically conservative, favoring political activism by physicians, holding a positive view of the social consequences of extrabilling, and perceiving family, associates, patients and the public to favor the strike.