The aim of this study was to evaluate changes in the peripheral blood CD(4)(+) and CD(8)(+) T-lymphocyte populations following strontium-89 chloride ((89)SrCl(2)) therapy for painful bone metastases and to relate these changes to a therapeutic response. Forty-two (42) patients with painful bone metastases were treated with 148 MBq (4 mCi) of (89)SrCl(2). Blood samples were drawn before and monthly for 6 months after the treatment. CD(4)(+) and CD(8)(+) T-lymphocyte levels were measured using flow cytometry. The number of bone metastases and the pain score were used to assess the effect of therapy. Before the administration of (89)SrCl(2), the ratio of CD(4)(+) to CD(8)(+) T-lymphocytes was lower in patients with bone metastases than in the control subjects (p < 0.01); after therapy, the ratio increased up to the fourth month and then gradually declined to pretreatment levels. Responders had higher post-therapeutic ratios of CD(4)(+) to CD(8)(+) than nonresponders. There was a good correlation between the ratio of CD(4)(+) to CD(8)(+) and both the number of bone metastases and the pain score. The ratio of CD(4)(+) to CD(8)(+) T-lymphocytes correlated strongly with the response of bone metastases to (89)SrCl(2), and therefore, may be used as an indicator of (89)SrCl(2) efficacy.