Two children with prolidase deficiency, an inborn error of proline met
abolism, developed clinical and immunological abnormalities consistent
with a diagnosis of systemic lupus erythematosus (SLE). The first chi
ld died from septicaemia, and SLE was only diagnosed during his termin
al illness. As a result of this diagnosis his cousin, who was already
known to have prolidase deficiency, was investigated further and a dia
gnosis of SLE confirmed. Following treatment with oral prednisolone he
r clinical condition has improved, although she has a persistently rai
sed erythrocyte sedimentation rate (ESR) and florid facial rash. Both
prolidase deficiency and SLE are associated with disturbances in immun
e function and have clinical features in common. It is Likely that pro
lidase deficiency is a risk factor for the development of SLE. Additio
nally, patients with SLE should-where there is a family history or pre
sentation in childhood-be specifically investigated for prolidase defi
ciency, since standard immunological or haematological investigations
will not identify the characteristic biochemical abnormalities.