An original psychometric instrument, the Multidimensional Inventory of Loneliness Experience (MILE-3), is presented, operationalizing an original theoretical model of personal attitudes towards loneliness. According to the model, acceptance of loneliness as an existential fact opens up a the possibility to of appreciatinge situations of solitude and to use their resources productively, in the process gaining a deeper self-awareness that promotes personal growth. Conversely, non-acceptance (or fear) of loneliness leads to an avoidance of solitude and to continuous search for social contacts, which results in avoidance of self-encounter and becomes an obstacle for personal growth. The MILE-3 contains 40 items (a short 24-item version is also available) grouped into 3 scales measuring the intensity of experienced loneliness and two opposite attitudes towards solitude and loneliness (positive solitude and dependence on communication). Three online samples were used (N=2298, N=144, and N=117). The structural validity of the instrument was established using structural equation modeling in the largest sample. The resulting scales were reliable (α in the .81-.90 range). Evidence of convergent and discriminant validity was provided by predictable associations with measures of subjective well-being (satisfaction with life, subjective happiness), life meaning, extraversion, affiliation motivation (need for acceptance and fear of rejection), and alienation. Using person-oriented approach methodology and cluster analysis, 4 distinct patterns of perceived loneliness were discovered and described.