The number of personal relationships occurring via the Internet is increasing as more people gain access to it. Many of these relationships are romantic in nature, and evidence is accumulating that they have the potential to have an adverse effect on existing face-to-face relationships. This study explored the formation of romantic relationships on their Internet, their nature, and their possible impact on existing marital or de facto relationships in a sample of 75 adults (mean age 42 years, SD = 11.1 years) who responded to an online survey of individuals involved in extradyadic relationships on the Internet. Respondents reported a variety of means of contacting their online partner. More females than males communicated with them daily. Most respondents knew what their partner looked like, most had contacted them by telephone, and a third had met them. Most reported more satisfaction with their online relationship than with their face-to-face one, though few said that it was more important to them than their primary relationship. Although only a quarter of the sample admitted that their online relationship had affected their primary one, those participants reported concealing the truth about the time or nature of their activities, that everyday tasks did not get done, and that levels of sexual intimacy with their primary partner had dropped. The nature of these and other problems suggests that therapists should be aware of the potential for Internet relationships to seriously affect face-to-face relationships.