Objective: We have investigated the effects of active and passive smoking on renal functions in terms of glomerular filtration rate, microalbuminuria, and beta-2 microglobulin excretion. Design and method: The volunteers included in this study were classified into three groups as active smokers (n = 24), passive smokers (n = 20), and controls (n = 20). Blood and urine samples were collected from all groups. Serum glucose, urea, creatinine, and cotinine levels in the collected blood samples were measured. Also, microalbumin, beta-2 microglobulin, and creatinine levels were measured in the collected urine samples. Results: Serum cotinine levels were found to be higher in both passive and active smokers when compared with controls (p < 0.01), whereas urinary microalbumin and creatinine levels were significantly higher in active smokers (p < 0.01). The urinary microalbumin/creatinine ratio was significantly increased in both active and passive smokers compared with controls. Conclusion: The kidney and the glomerular functions may be affected even by passive smoking. In addition, increased microalbumin/creatinine ratio may be a sign of increased atherosclerosis risk in these persons.