ObjectiveThis study aimed to quantify recall bias in the measurement of health-related quality of life (HRQoL), that is, the extent to which recollection is impaired and leads to distorted judgements.DesignProspective observational study.Setting and participantsOne hundred patients with two paradigmatic chronic diseases (50 with multiple sclerosis and 50 with psoriasis) were recruited at two outpatient clinics.Methods and outcome measuresPatients completed the online version of the 12-Item Short Form Survey (SF-12) repeatedly for 28 consecutive days: (1) daily, considering the past 24 hours; (2) weekly, considering the past 7 days; and (3) on the last day of data collection, considering the past 4 weeks. SF-12 scores for all three measurement approaches were subsequently converted into preference-based utility indices (Short-Form Six-Dimension). Agreement of the three indices was analysed on group and individual patient levels.ResultsThe mean age of participants was 40.3 years (±12.0), and 63% were female. The utility index based on daily recall (0.74±0.13) was more positive than indices based on a weekly (0.70±0.13, p<0.001) or a monthly (0.70±0.14, p<0.001) recall. While agreement of measurement approaches was high on group level (intraclass correlation coefficient>0.85), it was lower for the subgroup of patients experiencing high variability of HRQoL over time. Bland-Altman plots revealed considerable differences on individual patient level.ConclusionsOn the group level, retrospective overestimation and underestimation of HRQoL almost cancelled out one another and recall bias was relatively small. Therefore, a 4-week recall period could be appropriate when group-level data are used for research or economic evaluations. In contrast, recall bias can be considerable on the individual patient level and may thus impact decision-making in clinical practice.Trial registration numberVfD_RECALL_16_003837.