The clinical features of acute intermittent porphyria (AIP) are described in this chapter. AIP is inherited as an autosomal dominant pattern of inheritance. Prevalence in Japan is 1.5 in 100,000. Attacks are more frequent in women of 20s to 40s. The common clinical pattern of symptom involves acute abdominal pain, psychiatric disturbances, and acute neuropathy. The nerve biopsy shows segmental demyelination and axonal degeneration. Many small vacuolations are distinctively seen in all of the cell components of the nerve. Clinical diagnosis is not difficult when doctors keep the possibility of AIP in their minds in cases of abdominal pain, weakness and mental symptoms. The major trust of treatment is avoidance of acute attacks which is almost entirely dependent upon avoidance of porphyrogenic drugs. The intravenous administration of heme and glucose is important and effective therapy for acute attacks of porphyria.