OBJECTIVES: This study evaluated the rates of and reasons for treatment modifications and occurrence of adverse events (AEs) among patients treated with anti-angiogenic agents in UK clinical practice. METHODS: Data from medical records were retrospectively reviewed at 3 large UK oncology centers for mRCC patients who were ≥18 years and received sunitinib (N=90) as first-line treatment from January 1, 2005 to October 15, 2010.  Proportions of patients with treatment modifications (i.e.: discontinuation, interruption, or dose change) and reasons for modifications were determined.  Time to treatment discontinuation and proportion of patients with all grade and grade 3/4 AEs were also determined.  Data on clinician assessed response rates was collected. RESULTS:  Ten percent of patients were cytokine-pretreated.  Average daily dose over initial cycle was 31.98 mg; 77.8% of patients started treatment with recommended dosing of 50 mg QD 4/2.  Among the 62 patients with available tumor response assessments, 35.5% had an objective response (OR; complete or partial response).  A total of 84.4% of patients experienced AEs; 24.4% experienced grade 3/4 AEs.  The most commonly reported all grade AEs were diarrhea (35.6%), mucositis/stomatitis (34.4%), and fatigue (26.7%);  43.3% of patients had a dose reduction.  Adverse events led to treatment modifications in 60.0% of patients.  Among patients who discontinued treatment, 27.3% discontinued within 18 weeks (9.1% in 0-6 weeks, 6.1% in 7-12 weeks, and 12.1% in 13-18 weeks).  Among those who discontinued, 30.3% discontinued due to AEs. CONCLUSIONS: Objective response rate (35.5%) was higher than that observed in expanded access trial (17%) but lower than that observed in randomized clinical trial.  A large proportion (60%) of patients experienced treatment modifications due to AEs.  About 15.2% of treatment discontinuations occurred within the first two cycles.  This real-world clinical practice study suggests that tolerability with sunitinib is a challenge for physicians in the clinical care of mRCC patients in the UK.