Folic acid fortification of grain products was mandated in the USA by January 1998 and in Canada by November 1998. It was hypothesized that screening total plasma homocysteine levels adjusted for age, sex, race and country that were drawn in stroke patients for the Vitamin Intervention for Stroke Prevention trial from 1997 to 2001 would be steady when fortification was completed. Samples were grouped by years 1997/1998, 1999 and 2000/2001, and adjusted means were calculated using a general linear regression model. In 2,612 US and 1,059 Canadian patients, levels showed no consistent trend in the USA alone, while Canadian levels declined (p = 0.06 overall, 0.0003 in the oldest age group). US levels were 0.39 μmol/l (95% CI: -0.08, 0.85) lower than in Canada. Neither fasting nor time since stroke affected the results. Copyright © 2006 S. Karger AG.