Objectives. We analyzed the content and characteristics of influential health blogs and bloggers to provide a more thorough understanding of the health blogosphere than was previously available. Methods. We identified, through a purposive-snowball approach, 951 health blogs in 2007 and 2008. All blogs were US focused and updated regularly. We described their features, topics, perspectives, and blogger demographics. Results. Approximately half of the bloggers in our sample were employed in the health field. A majority were female, aged in their 30s, and highly educated. Two thirds posted at least weekly; one quarter accepted advertisements. Most blogs were established after 2004. They typically focused on bloggers' experiences with 1 disease or condition or on the personal experiences of health professionals. Half were written from a professional perspective, one third from a patient-consumer perspective, and a few from the perspective of an unpaid caregiver. Conclusions. Data collected from health blogs could be aggregated for largescale empirical investigations. Future research should assess the quality of the information posted and identify what blog features and elements best reflect adherence to prevailing norms of conduct.