Purpose <p>Understanding how to optimally measure symptoms and wellbeing of people with advanced cancer is crucial for supporting patient-centered care. We aimed to (1) compare repeated in-the-moment assessments (experience sampling methods/ESM) and retrospective assessments (traditional patient-reported outcome measures/PROMS) of symptoms and well-being among people with advanced breast or lung cancer; and (2) explore factors associated with discrepancies between these methods.</p> Methods <p>In an observational study among people with advanced breast or lung cancer, participants completed up to 60 in-the-moment ESM assessments over 7 days, followed by a 7-day recall (i.e., retrospective) questionnaire covering the same period. We compared in-the-moment and retrospective scores of 16 symptom and wellbeing items visually and through correlations. We examined factors associated with discrepancies using linear regression.</p> Results <p>We analyzed 1676 in-the-moment assessments from 36 participants. Visually, higher in-the-moment scores were associated with higher retrospective scores across the sample. But, participants with identical retrospective scores often had different means (especially when they had higher recalled scores) and fluctuation patterns of in-the-moment scores. Item correlations between in-the-moment and retrospective scores ranged between .24 and .70. The largest discrepancies occurred for pain (<i>Mdiff</i> = −13.2) and tiredness (<i>Mdiff</i> = −12.4). Several parameters of in-the-moment scores and participants’ active treatment status were associated with discrepancies.</p> Conclusion <p>Individuals’ retrospective symptom and wellbeing scores positively correlated with their in-the-moment scores over 1 week. Pain and tiredness showed the largest discrepancies. In-the-moment scores revealed considerable variability between individuals and fluctuations over time, which may be relevant to assess depending on the clinical or research objective.</p>