Gynaecomastia is a common clinical condition. Persistent pubertal or late onset idiopathic gynaecomastia is the leading cause of gynaecomastia in different series. The aim of this study was the assessment of the prevalence and characteristics of different causes of gynaecomastia in young adult males, and evaluation of the factors associated with idiopathic gynaecomastia. Fifty-three male patients (mean age 22.04 +/- 2.22, range 19-29), who had been admitted to our outpatient clinics with gynaecomastia as the main presenting symptom were enrolled in the study. Patients were evaluated with breast palpation, breast ultrasonography, anthropometric measurements and sex steroid levels. Secondary causes of gynaecomastia were ruled out. Thirty age-matched healthy individuals were also studied as healthy control group. Idiopathic gynaecomastia was diagnosed in 31 of 53 patients (58%), with 17 (32%) persistent pubertal and 14 (24%) late onset course. Other causes of gynaecomastia were hypogonadism in 13 cases (25%), hyperprolactinaemia in five (9%), chronic liver disease in two (4%), and drug induced (prolonged use of H2 antagonists) in two (4%). Patients with idiopathic gynaecomastia, either pubertal or late onset, were compared with the healthy control group in order to find out associated factors. Anthropometric measurements revealed a significant increase in body weight and body mass index (BMI) in the patient group compared with healthy controls (72.4 +/- 13.3 vs. 63.6 +/- 7.9 kg, p = 0.0086 and 25.2 +/- 4.0 vs. 21.5 +/- 2.7 kg/m2, p = 0.0001). Total skin fold thickness (SFT) of four different regions were also higher in the patient group (50.9 +/- 22.1 vs. 32.6 +/- 10.2 mm, p = 0.0006) indicating a higher body fat percentage. Total serum testosterone (4.76 +/- 1.31 vs. 5.70 +/- 1.06 microg/mL, p = 0.0038) and luteinizing hormone (LH) (4.80 +/- 1.92 vs. 7.32 +/- 1.90 mIU/mL, p < 0.0001) levels were significantly lower in the patient group while oestradiol levels were similar. There was a significant correlation between total testosterone and LH levels (r = 0.27, p = 0.0445). Total testosterone and LH levels were negatively correlated with BMI and total SFT. As a result most common form of gynaecomastia is idiopathic gynaecomastia either as persistent pubertal or late onset forms in young adult males. Idiopathic gynaecomastia is closely correlated with generalized obesity, reduced LH and testosterone levels which may be the result of increased conversion of testosterone to oestradiol in increased adipose tissue mass.