Multivariate genetic analyses were used to examine the genetic and environmental contributions to individual differences in fears of blood, injury, and injections in 659 twin pairs who completed questions concerning fear and fainting around blood, injury, and injections, and fainting in situations not involving blood, as well as the personality scales of Neuroticism, and Harm Avoidance. There was significant familial aggregation of blood fears but univariate analyses were unable to distinguish between additive genetic or shared environmental variables, or both, as the cause. The same was true of blood fainting. Non-bloodinjury fainting was best explained by a model assuming shared and unique environmental variables. However, multivariate genetic analyses, which capitalise on extra information contained by all the covariance terms, indicated that the variance in bloodinjury-injection fear was principally attributable to unique environmental events specific to this fear and additive genetic factors shared with fainting. The data are discussed in the context of models of blood-injury phobia that identify the need to consider separate etiological mechanisms for fear and fainting. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 81:377-384, 1998.