Little is known about the quality and usability of on-line health information. This analysis evaluated smoking cessation Web sites' content quality and usability. Thirty sites were analyzed to determine their adherence to established tobacco cessation guidelines and their accessibility, usability, credibility, and currency. Most explained addiction (86.7%) and mentioned nicotine replacement therapy (NRT) (93.3%) and social support (93.3%). However, few explained potential side effects of NRT (33.3%) or which smokers should avoid using NRT (30.0%). Two sites advocated substituting smokeless tobacco or herbal cigarettes when quitting, and 16 (53.3%) provided information written at greater than an eighth-grade level. Few sites provided a search mechanism (40.0%) or offered text-only versions (30.0%), and most (83.3%) failed to indicate when content pages were last updated. Most sites adhered to established cessation guidelines. A small subset offered erroneous and potentially harmful information. Applying fundamental design principles would improve accessibility, usability, credibility, and currency.