Piroxicam and naproxen plasma concentrations were obtained after 4 weeks active therapy between 3 and 12 hours post-dose in 640 and 629 patients, respectively. These patients are a subset of 2,035 patients with osteoarthritis on whom we have reported previously (11) in a double-blind multicentre safety and efficacy trial comparing piroxicam 20 mg/day and naproxen 750 mg/day. The purpose of the present study was to look for an association of plasma drug concentration with the variables of: age, sex, adverse events and efficacy. There was a statistically significant increase in plasma concentrations of both drugs with increasing age and females had higher concentrations than males. The increase in plasma concentration seen with increasing age was of a magnitude of 25% for piroxicam and 20% for naproxen when comparing a 50 year old to an 80 year old. However, less than 15% of the variability in plasma concentrations seen between patients is accounted for by age and sex. Within the plasma concentrations achieved with these doses, no association with adverse events, non-serious or serious, and efficacy was noted.