What has been the effect of giving tiny amounts of mercury to millions of children in vaccines? A huge study in the November 2003 Pediatrics used an analysis of the medical records of over 140,000 children to try to find an answer. Health maintenance organization (HMO) computer databases make this kind of analyses possible. Children in three different HMOs were followed, from 1991 to 1999. Researchers tracked cumulative mercury (thimerosal) exposure and health outcomes. In HMO A, those with the highest cumulative mercury exposure at 3 months of age had a 1.89 times higher chance of later developing tics than those with lower exposures. But in HMOs B and C, this didn't happen. However, in HMO B, those children who had the highest cumulative exposure at 3 months had 1.13 times the risk of speech delay of their peers, and those who had the highest cumulative exposure at 7 months had 1.07 times the risk of speech delay. But this didn't happen in HMOs A and C. Indeed, in HMO C, no significant associations were found. And in none of the groups was there any higher risk of autism or attention deficit hyperactivity disorder (ADHD). It is reassuring that potential associations with increased risk of health problems were small and inconsistent. Nevertheless, I continue to be glad that today all of the routine vaccines for children, including the flu shot, are available in forms without added thimerosal.