Two children with prolidase deficiency, an inborn error of proline metabolism, developed clinical and immunological abnormalities consistent with a diagnosis of systemic lupus erythematosus (SLE). The first child died from septicaemia, and SLE was only diagnosed during his terminal illness. As a result of this diagnosis his cousin, who was already known to have prolidase deficiency, was investigated further and a diagnosis of SLE confirmed. Following treatment with oral prednisolone her clinical condition has improved, although she has a persistently raised erythrocyte sedimentation rate (ESR) and florid facial rash. Both prolidase deficiency and SLE are associated with disturbances in immune function and have clinical features in common. It is likely that prolidase deficiency is a risk factor for the development of SLE. Additionally, patients with SLE should-where there is a family history or presentation in childhood-be specifically investigated for prolidase deficiency, since standard immunological or haematological investigations will not identify the characteristic biochemical abnormalities. (Arch Dis Child 1997;76:441-444)