- Waldenström macroglobulinemia (blood cell disease)
- Warfarin (Coumadin) use
- Warfarin (Coumadin, Jantoven) overdose
- Wegener's granulomatosis (granulomatosis with polyangiitis, blood vessel disease)
- West Nile virus infection (mosquito-transmitted illness)
- Whiplash injury (neck injury)
- Whipple's disease (rare bacterial infection of the small intestine)
- White coat hypertension (elevated blood pressure at doctor's office)
- Whooping cough (pertussis)
- Wilms' Tumor
- Wilsons disease (copper accumulation in tissues)
- Windshield wiper fluid (Methanol) poisoning
- Wolff Parkinson White syndrome (WPW, additional electrical pathway in the heart)
- Wood alcohol (methanol) poisoning
- Wound infection (cut infection)
- Wrist sprain