Research on human subjects analyzing blood and urine samples determined biological correlates that may explain the pathology of alcohol hangover. These analyses showed that concentrations of various hormones, electrolytes, free fatty acids, triglycerides, lactate, ketone bodies, cortisol, and glucose were not significantly correlated with reported alcohol hangover severity. Also, markers of dehydration (e.g., vasopressin) were not significantly related to hangover severity. Some studies report a significant correlation between blood acetaldehyde concentration and hangover severity, but most convincing is the significant relationship between immune factors and hangover severity. The latter is supported by studies showing that hangover severity may be reduced by inhibitors of prostaglandin synthesis. Several factors do not cause alcohol hangover but can aggravate its severity. These include sleep deprivation, smoking, congeners, health status, genetics and individual differences. Future studies should more rigorously study these factors as well as biological correlates to further elucidate the pathology of alcohol hangover.