Neuroporphyrias, a heterogeneous group of metabolic diseases, are diagnosed less often than their true prevalence justifies. Lack of awareness of porphyrias and their protean clinical and biochemical manifestations, is the most significant hurdle to their recognition and diagnosis. These points are reflected in the unusual case reported here, which highlights the potential damage that inappropriate management may cause when the diagnosis is missed over a long period. We diagnosed heterozygous Acute Intermittent Porphyria (AIP) in a 15 yr old girl, who first presented with autism at the age of 4 years. This phenotypic association has not been previously reported. In addition to the unrecognized phenotype, her normal urinary aminolevulinic acid and porphobilinogen, findings which are not compatible with symptomatic porphyria according to well established criteria, could also have led to a missed diagnosis of neuroporphyria. However, the diagnosis of AIP was established on the basis of a 64% reduction in erythrocyte hydroxymethylbilane synthase (HMBS) activity and the finding of a known causative AIP mutation (p.D178N). We therefore recommend that porphyria should be considered in autistic children especially when there is an atypical course or unexpected abreaction to medications. The biochemical and genetic data should be carefully evaluated in a specialized porphyria center.