A 78-year-old man with metastatic prostate cancer was referred to the hospital 5 weeks after the initiation of systemic therapy with goserelin (GnRH agonist) because of a significant increase in alkaline phosphatase (ALP) concentration despite clinical improvement. Further workup revealed a decrease in prostate-specific antigen levels and a lack of radiological signs of disease progression. Subsequently, the ALP dropped spontaneously. This case report is an example for an early ALP flare after initiation of endocrine therapy in patients with bone metastasis which is consistent with a treatment response. Clinicians should be familiar with the ALP flare phenomenon in this setting, which does not reflect disease progression or treatment failure, in order to prevent unnecessary investigations, hospital admissions, or even erroneous termination of successful therapy.