Within hematology and oncology, companion diagnostics (CDxs) play an increasing role in securing an optimal therapy for individual patients, and the US Food and Drug Administration (FDA) consider this type of assay essential for the safe and effective use of a corresponding therapeutic product. Most CDxs are developed prospectively using the drug-diagnostic codevelopment model, which normally secures the simultaneous approval of both drugs and diagnostics. A CDx assay is an important treatment decision tool that needs to be available simultaneously with the drug. However, within the past few years, several targeted drugs and new indications have been approved by the FDA without a CDx, despite the use of a predictive biomarker assay for patient selection during clinical development. A missing analytical and clinically validated CDx assay could affect the correct use of these drugs and ultimately patient safety. An alternative to FDA-approved or FDA-cleared CDxs could be to use a laboratory-developed test, which will normally miss documentation on the clinical validity. On the basis of the information available from different publicly available FDA databases, this article briefly discusses the issue of missing CDx assays in relation to the approval of hematological and oncological drugs and new indications.