Malignant bone disease can cause significant morbidity. Monthly zoledronic acid (ZOL) reduces skeletal complications; however, limited data are available regarding long-term safety. We aimed to assess efficacy and safety of ZOL beyond 1 year of treatment. We prospectively evaluated 73 patients; breast cancer (n = 29), castrate-resistant prostate cancer (n = 13), multiple myeloma (n = 31) from 2006 to 2008 in 19 cancer centres. All patients were diagnosed with bone disease and had completed 1–2 years of monthly ZOL (4 mg) and received a further 1–2 years of therapy following contemporary guidelines for managing risks of osteonecrosis of the jaw (ONJ) and renal toxicity. Overall rates of skeletal-related events (SREs), renal impairment and ONJ were assessed. Over the additional 1 year of treatment, only 5.5% (n = 4) of patients developed a new SRE. The overall Kaplan–Meier estimate for SRE incidence after 48 weeks on study was 6.75% (95 CI: 2.5–17.3). Although 51% of patients reported serious adverse events, only two cases were suspected as ZOL related. No patients had confirmed ONJ. The observed incidence of new renal impairment was 11% (none due to ZOL). Our study confirms the benefit over risk of continuing monthly ZOL for at least 2 years in patients with advanced cancer involving bone.