Loneliness, a complex phenomenon, is experienced differently by people under diverse conditions. The factors influencing loneliness are numerous and may vary between cultures. The purpose of this study was to show socio-demographic factors contributing to loneliness in Portugal. The sample consisted of 3,144 participants with a mean age of 46.90 (SD = 22.56) and a range between 15 and 92 years. Loneliness was evaluated with the Revised UCLA Loneliness Scale (Russell, Peplau, & Cutrona, 1980). Age and marital status were significant predictors of loneliness. Loneliness increased with age, and divorced or windowed participants reported higher loneliness than single or married people. As expected, gender did not significantly contribute to loneliness. This work identified vulnerable people who are experiencing a perceived dissatisfaction with their social interactions that needs special attention. These vulnerable groups include the old, divorced and widowed. In particular, policy makers and other experts who work with old persons should promote interventions according to their needs.