The effects of celiprolol on fasting plasma leptin levels, glucose tolerance, and insulin sensitivity were studied in a randomized, investigator-masked, and parallel clinical trial. Modified oral glucose tolerance tests (OGTT) were performed during the previous antihypertensive monotherapy (beta- or Ca-blocker, or ACE inhibitor), and 6 and 12 months after randomization to celiprolol (200-400 mg daily) or to control group, where the therapy was kept unchanged. One hundred sixty-nine dyslipidemic and hypertensive nondiabetics with an age range of 42-65 years and an average body mass index of 28.4 kg/m2 completed the study according to the protocol. The mean circulating leptin level decreased from 7.5 to 6.6 ng/mL in men (p < 0.05) and from 23.0 to 19.7 ng/mL in women during the 12-month celiprolol treatment. The incremental glucose area under the curve (AUC) in the 2-hour OGTT decreased from 3.8 to 3.0 h* mmol/L (p < 0.01), and insulin AUC decreased from 134 to 99 h* mU/L (p < 0.01). The insulin sensitivity index increased by 22% (p < 0.01) and the serum triglyceride level decreased by 15% in the celiprolol group. Changes in serum cholesterol were clinically insignificant. In the control group, no significant change was seen in any measured variable. A decrease in leptin levels in the celiprolol group was associated with improved insulin sensitivity, while the weight of the moderately obese patients did not change. The clinical significance of a 14% decrease in fasting plasma leptin level remains to be elucidated. The results suggest amelioration of leptin resistance during long-term celiprolol therapy.