The relevance of quantitative determinations of urinary deoxypyridinolines (DPY) and pyridinolines (PY), and of serum type I collagen carboxyterminal cross-linked telopeptides (ICTP), has been evaluated for patient monitoring in multiple myeloma (MM). In 178 untreated MM patients, a clear correlation was found between ICTP concentrations, bone destructions and serum calcium levels. Furthermore, serum ICTP, urinary DPY and PY concentrations were estimated before and during treatment in a further 33 MM patients randomly allocated to four groups receiving intravenous melphalan/prednisone (MivP) chemotherapy alone, or MivP in combination with three different doses of i.v. clodronate. 1800 mg of i.v. clodronate combined monthly with MivP induced a rapid and sustained reduction in bone resorption parameters to the normal range, a result not obtained with either MivP alone, or with a lower clodronate dose. While confirming the relevance of determining pyridinium cross-links for estimating bone resorption in MM, our data indicate that measurements of these parameters could be useful for dose finding and monitoring of bisphosphonate therapy.