Δ~1-pyrroline-5-carboxylate synthase (P5CS) catalyses the reduction of glutamate to Δ1-pyrroline-5-carboxylate, a critical step in the biosynthesis of proline, ornithine and arginine. Recently, we reported a newly recognised inborn error due to deficiency of P5CS in two sibs, one presenting at birth with hypotonia, dysmorphic signs, pes planus and clonic seizures. Both developed progressive neurodegeneration and peripheral neuropathy, joint laxity, skin hyperelasticity and bilateral subcapsular cataracts. Their metabolic phenotype includes mild hyperammonaemia, hypo-ornithinaemia, hypocitrullinaemia, hypo-argininae- mia and hypoprolinaemia. Incorporation of 3Hproline into protein was deficient in fibroblasts incubated with 3H-glutamate. Both patients are homozygous for the missense mutation R84Q in P5CS. Here, we describe the clinical phenotype of the sibs in detail and show that a relative deficiency of urea cycle intermediates (ornithine, citrulline and arginine) during fasting periods results in a paradoxical hyperammonaemia. Furthermore, we show the results of ornithine loading tests and indirect enzyme studies corroborating the biological signifi-cance of the defect in P5CS in vivo. Conclusion: Themetabolic phenotype of Δ1-pyrroline-5-carboxylate synthase deficiency is easily missed. The combination of low levels of ornithine, citrulline, arginine and proline plus a tendency to hyperammonaemia or one of the above together with a clinical phenotype of neurodegeneration with peripheral neuropathy and/or cataracts and connective tissue manifestations should suggest this disorder. Early recognition would allow a therapeutic trial with citrulline and proline. A ~ 1-pyrroline-5-carboxylate synthase (P5CS) catalyses the reduction of glutamate to Δ1-pyrroline-5-carboxylate, a critical step in the biosynthesis of proline, ornithine and arginine. Recently, we reported a newly recognized inborn error due to deficiency of P5CS in two sibs, one presenting at birth with hypotonia, dysmorphic signs, pes planus and clonic seizures. Both developed progressive neurodegeneration and peripheral neuropathy, joint laxity, skin hyperelasticity and bilateral subcapsular cataracts. Their metabolic phenotype includes mild hyperammonaemia, hypo Both of them are homozygous for the missense mutation R84Q in P5CS. Here, we describe the clinical phenotype of the sibs in detail and show that a relative deficiency of urea cycle intermediates (ornithine, citrulline and arginine) during fasting per iods results in a paradoxical hyperammonaemia. Moreover, we show the results of ornithine loading tests and indirect enzyme studies corroborating the biological signifi-cance of the defect in P5CS in vivo. Conclusion: Themetabolic phenotype of Δ1-pyrroline-5-carboxylate synthase deficiency is easily missed. The combination of low levels of ornithine, citrulline, arginine and proline plus a tendency to hyperammonaemia or one of the above together with a clinical phenotype of neurodegeneration with peripheral neuropathy and / or cataracts and connective tissue manifestations should suggest this disorder. Early recognition would allow a therapeutic trial with citrulline and proline.