Data on unreasonable fears of blood, needles, hospitals, and illness (BNHI) were collected by telephone interview from 541 MZ and 388 DZ pairs of female twins from the population-based Virginia Twin Registry. BNHI phobia was defined as the presence of fear accompanied by interference. Age at onset of phobia was found to be very similar to that of situational phobias previously assessed in the sample. Using a multiple threshold model, we found no evidence for qualitative differences between BNHI fears and BNHI phobia. The familial aggregation of fears appears to be entirely due to additive genetic variance. The possible exception to this is fear of illness, which, like BNHI phobias, seems to aggregate within families because of shared environmental factors. Although power to discriminate between the causes of familial resemblance is low, results suggest that random traumatic events and some social learning may be responsible for the onset of BNHI phobias. About two-thirds of variance is individual-specific environmental, and could include genotype x environment interaction and measurement error.