A survey conducted by Knowledge Networks on a panel that includes both internet users and non-users, shows that "[a]pproximately 40% of respondents with Internet access reported using the Internet to look for advice or information about health or health care in 2001. […] About one third of those using the Internet for health reported that using the Internet affected a decision about health or their health care, but very few reported impacts on measurable health care utilization."

This reported usage is apparently lower than what other similar studies had shown in the past, though it still looks significant to me.

The results of this study have been published in the Journal of the American Association, you can get an abstract for free and the full text is available for a fee. (Story lead thanks to MediaPost.)