Changes in the serial measurements of serum prostatic acid phosphatase (PAP), and prostatic specific antigen (PSA) have been compared against changes in serial bone scans in 120 patients with prostatic cancer. Of 54 patients who presented with negative bone scans 10 developed skeletal metastases, the PAP and PSA levels were rising in 5 and 9 of these patients, respectively. Local progression occurred in a further 9 patients in whom PAP was rising in 8 and PSA in all 9. In the 66 patients with previously documented skeletal metastases bone scan evidence of progression was seen in 36. At the time of the first evidence of progression PAP was rising in 20 (55%) and PSA was rising in 26 (72%). In 4 patients neither marker was raised at the time of first evidence of progression. We discuss the value of ‘routine’ serial bone scintigraphy in monitoring patients with prostatic cancer.