Renal function aberrations during bisphosphonate treatment is a well-known phenomenon. In our retrospective study we examined renal functions of 97 breast cancer patients with bone metastasis during their first year of bisphosphonate treatment i.e. (1) frequency of initial renal function alterations; (2) frequency of decreasing renal function during bisphosphonate treatment; (3) the connection between the laboratory findings and the renal function parameters measured at the beginning of bisphosphonate treatment. At the beginning of bisphosphonate treatment we found a surprisingly high rate (26.80%) of decreased creatinine clearance calculated by the Cockcroft-Gault formula. Decreased creatinine clearance at least once during bisphosphonate treatment has been found in 32.99% of the patients, and in 13.4% of the patients with decreased renal function parameters before bisphosphonates it remained decreased during the one-year period. Expected normal renal function is prognosticated by the renal function parameters and serum calcium level measured before starting bisphosphonate treatment. However, we could not demonstrate any connection between decreasing renal function and either the route of administration or the generation or type of bisphosphonates or the previous use of platinum compounds. Our analysis confirms the necessity of monitoring renal function before and during bisphosphonate treatment, and it is advisable to calculate the creatinine clearance in the upper quarter of the normal range of creatinine levels. In case of decreased renal function, change to a less nephrotoxic bisphosphonate or discontinuing the treatment is suggested. While our results are at variance with the published literature, the above-mentioned questions should be examined in a prospective trial.