ALAD porphyria is an autosomal recessive disorder resulting from a homozygous aminolevulinic acid dehydratase (ALAD) deficiency. Because of an almost complete lack of ALAD activity, patients excrete a large amount of ALA, but not PBG, into urine. The symptoms in this disease are similar to those seen in AIP, but ALAD porphyria can be differentiated from AIP by its autosomal recessive, rather than dominant, inheritance, by the lack of PBG overproduction, and by markedly decreased ALAD activity.