There are few cross-cultural studies utilizing longitudinal analysis to explore problematic internet use (PIU), and almost none among adults. The present follow-up study compared three waves across 12-month period every six months and observed the natural course and trajectory of PIU in a European multi-country sample of adults from 11 countries (Finland, Germany, Italy, Spain, France, Switzerland, Hungary, Poland, UK, Norway, Belgium). A total of 139 participants (45.5% females) provided data across all three waves with an average age of 26.14 years (SD = 5.92). There were longitudinal effects in PIU, with statistical differences between at-risk users compared to healthy users in Waves 1 and 2, and Waves 1 and 3. The analyses of variance showed a longitudinal effect of waves on the PIU symptoms. PIU was significantly affected by time and type of user, with those classed as at-risk having higher scores than healthy users, although PIU decreased over time. In addition, the type of PIU detected in adults contained mild addictive symptoms. In conclusion, this study demonstrated that PIU was generally low among European adult population and tended to decrease over the one-year period, what contrasts with adolescent population findings.