Sir: We have read the interesting article by Pietschmann et al about the serum osteocalcin concentrations in patients with rheumatoid arthritis.' In their work there was no mention of the time at which blood samples were taken for osteocalcin determination. Several studies have shown a circadian rhythm of serum osteocalcin in normal adults, with peak values during the night and a nadir during the morning hours.2 3 Therefore, in our opinion, if osteocalcin is used as a marker in clinical investigations of bone metabolism it is important to mention the time at which blood was collected for its measurement. Otherwise, interpretation of results may be difficult and comparison with values obtained in other studies impossible.