Medical Sciences. In the article “Osteopontin-deficient mice are resistant to ovariectomy-induced bone resorption” by Hiroyuki Yoshitake, Susan R. Rittling, David T. Denhardt, and Masaki Noda, which appeared in number 14, July, 1999, of Proc. Natl. Acad. Sci. USA (96, 8156–8160), the authors request the following correction. In the 9th line in the first paragraph and in Table 3 on page 8158, the unit in the text and the third subheading under Parameters in the table are incorrectly listed as BFR/BV, μm³/μm²/day. The correct listing is BFR/BV, %/year.