Postabsorptive urine hydroxyproline (HYPRO) excretion (spot HYPRO test) was compared with the 24 h urine excretion of HYPRO in 45 patients with breast cancer and 9 normal subjects on a low-gelatin diet. A good correlation was observed between the results of these 2 tests in both groups. Patients with skeletal metastasis showed a higher spot HYPRO value than those without bone involvement and the number of lesions by bone scanning correlated with the values of spot HYPRO. In 20 of 22 patients with bone metastasis followed up for 3-19 mo., spot HYPRO values correlated well with the evolution of bone disease. The spot HYPRO test is a simple, inexpensive and accurate method for the diagnosis and follow-up of patients with skeletal metastasis from breast cancer.