Progressive multifocal leukoencephalopathy (PML) has been identified as a serious adverse drug reaction (ADR) of several immunomodulatory biologicals. In this study, we contrasted the reporting patterns of PML for two biologicals for which the risk was identified at different points in their lifecycle: natalizumab (before reapproval) and rituximab (nine years postapproval). We found that, apart from the differences in clinical characteristics (age, gender, indication, time to event, fatality), which reflect the diversity in context of use, PML reports for natalizumab were more complete and were received sooner after occurrence. This study serves as an important reminder that spontaneous reports should only be used with great caution to quantify and compare safety profiles across products over time. The observed variability in reporting patterns and heterogeneity of PML cases presents challenges to such comparisons. Lumping uncharacterized PML reports together without taking these differences into account may result in biased comparisons and flawed conclusions about differential safety.