To identify situations and thoughts that may precipitate or protect against loneliness experienced by patients with cancer. 
 Qualitative. 
 The hematology/oncology clinic at the Indiana University Melvin and Bren Simon Cancer Center, an outpatient oncology center in Indianapolis. 
 Purposive sample of 15 patients undergoing treatment for multiple myeloma or non-Hodgkin lymphoma. 
 Individual, semistructured qualitative interviews were conducted. Theoretical thematic analysis was used to analyze interview data.
 Factors that appeared to precipitate loneliness included several situations (e.g., physical isolation, social constraints such as criticism) and thoughts (e.g., unmet expectations for visits or questions about health, belief that others do not understand their cancer experience). Several situations (e.g., social support, normal routine) and thoughts (e.g., beliefs that time alone is desirable and that others' discomfort with cancer-related discussions is normative) appeared to protect against loneliness. Certain social situations were loneliness-inducing for some patients and not for others, suggesting that patients' thoughts about their situations, rather than the situations themselves, have the greatest impact on their loneliness. 
 The current study fills gaps in loneliness theory by identifying cancer-related situations and thoughts that patients associate with their loneliness. Consistent with theory, patients reported feeling lonely when they had negative thoughts about their social situations. 
 Findings inform nursing assessment and intervention strategies to incorporate into care plans. For instance, when conducting assessments, nurses should be more attentive to patients' satisfaction with their social environment than actual characteristics of the environment. Normalizing patients' experiences and encouraging positive thoughts about others' behavior may reduce patients' loneliness.