Hyperekplexia or startle disease was diagnosed in four patients, a girl of 14 months, two men of 45 and 61 years old, and a woman of 56 years old. This is an autosomal dominant inherited disorder, characterized by excessive startle reactions, notably to auditory stimuli. The four patients belong to a Dutch family in which two forms of the disorder occur. The first one is the major form, in which the startle reaction is followed by a temporary generalized stiffening of the body. These patients experience continuous generalized stiffness from birth to about the second year of life. They fall down frequently. The other is the minor form, characterized by an excessive startle reaction only.