Objective: To identify the genetic etiology of a distinct leukoencephalopathy with autosomal recessive inheritance in a single family. Methods: We analyzed available MRIs and retrospectively reviewed clinical information and laboratory investigations. We performed whole-exome sequencing to find the causal gene variants. Results: We identified 3 family members with a similar MRI pattern characterized by symmetrical signal abnormalities in the periventricular and deep cerebral white matter, thalami, and central part of the pons. Cerebellar atrophy was noted in advanced disease stages. Clinical features were childhood-onset slowly progressive spastic paraparesis, cerebellar ataxia, peripheral neuropathy, and in 2 patients, optic atrophy as well as vertical gaze and convergence palsies and nystagmus. Whole-exome sequencing revealed compound heterozygous missense variants in the HMBS gene, both associated with the autosomal dominant disorder acute intermittent porphyria. Sanger sequencing of 6 healthy siblings confirmed the bi-allelic location of the variants and segregation with the disease. Patients had a slight and moderate increase in urinary and plasma porphobilinogen and 5′-aminolevulinic acid, respectively, and a 50% to 66% decrease in hydroxymethylbilane synthase enzyme activity compared to normal. Conclusions: Bi-allelic HMBS variants have been reported before as cause of severe encephalopathy with early childhood fatality in acute intermittent porphyria. Our cases demonstrate childhood onset, but milder and slower disease progression in middle-aged patients. With this, a novel phenotype can be added to the disease spectrum associated with bi-allelic HMBS variants: a leukoencephalopathy with early onset, slowly progressive neurologic symptomatology, and long life expectancy.