Systemic prostaglandin E-M (PGE-M) in the urine before and after naproxen treatment in all patients. The urinary PG profile was determined prior to, after 8 weeks of treatment and after another 8 weeks of cessation of treatment with the NSAID naproxen (15 mg/kg of an oral suspension per day). Treatment cycles were repeated according to clinical necessity, especially when the patient's parents reported significant physical impairment due to pain. The levels of PGs and their metabolites were determined in 24-hour urine samples by capillary gas chromatography and mass spectrometry. Systemic prostaglandin PGE-M production was evaluated in 15 treatment cycles (connected lines). Eight out of 15 revealed elevated levels above the normal range before the start of therapy. All PGE-M levels dropped into the normal range immediately after therapy (paired t-test, p = 0.0012). In addition, subsequent PGE-M measurements taken 8 weeks after treatment cessation showed a re-elevation in PGE-M levels (paired t-test, p = 0.0011).