From September 2007 to May 2011 a total of 471 participants (325 males and 146 females) signed up for an 8-week Internet-based cognitive behavioral therapy offered for gamblers in Finland. Sixty-four percent of the participants were pathological gamblers (PGs) (NODS 5> points), 14% were problem gamblers (NODS 3-4 points) and 10% were at risk of gambling problems (NODS 1-2 points). Two hundred and twenty four participants completed the treatment and after the treatment period significant changes were found in the following variables: gambling related problems (NODS), gambling urge, impaired control of gambling, alcohol consumption (AUDIT-C), social consequences, gambling-related cognitive erroneous thoughts and depression (MARD-S). In this sample co-morbid alcohol consumption was stronger among males. The main finding of this study was that the onset age of gambling was associated with a greater amount of gambling-related cognitive erroneous thoughts.