<jats:title>Abstract</jats:title><jats:p>Three patients with ketosis had increased excretion of 3‐hydroxyglutarate (21.8–37.9 μmol/mmol creatinine; controls 2.3 ± 1.6), an indicator of glutaryl‐CoA dehydrogenase deficiency (GDHD), which normalized when the patients were nonketotic. Clinical assessment of all three patients and enzyme studies in one patient were not consistent with GDHD. These findings were compared with those of other ketotic patients, who showed statistically significant increases in 3‐hydroxyglutarate excretion (9.4 ± 5.0 μmol/mmol creatinine; <jats:italic>p</jats:italic> < 0.01), and with those of a child with confirmed GDHD when she was both ketotic and nonketotic. Secondary increase in 3‐hydroxyglutarate excretion during ketosis is a potential confounder in the diagnosis of GDHD.