OBJECTIVE Various targeted disease-specific therapeutics are currently approved, demonstrating a survival benefit over therapy with interferon-alpha (IFN-α) in patients with metastatic renal cell carcinoma (mRCC). Temsirolimus, a highly specific inhibitor of the mammalian target of rapamycin (mTOR), improves the overall and progression-free survival of high-risk patients with mRCC. The purpose of this study was to estimate the effects of temsirolimus on several laboratory parameters and to report the potential adverse events (AEs) in patients with mRCC. METHODS This research was a controlled, open, prospective and partly retrospective randomized study that included 60 patients up to 65 years of age, divided into the experimental and control group, each containing 30 patients. Patients in the experimental group were treated with temsirolimus. The control group comprised patients in the same stage of disease, treated with IFN-α. The effect of therapy in both groups was monitored during the first year of administration. RESULTS The overall incidence of AEs was 40% in both groups. Sixteen laboratory parameters were analyzed and the total number of deviations from the reference range was 263 in the experimental group and 229 in the control group. The total number of AEs regarding patient general clinical condition in the experimental group was 193 (asthenia 53.3%, urinary infection 43.3% and pyrexia 40%) and 175 in the control group (pyrexia 76.7%, asthenia 50% and tremor 50%). CONCLUSIONS Monitoring the renal function parameters during the temsirolimus administration has proved that the therapy had no significant influence on the remaining kidney function. By evaluating the AEs we concluded that there was no significant difference in the number of AEs of all grades between the groups, while the laboratory parameters and physical status deterioration differed qualitatively.