The concept that androgen alone can provide an effective male contraceptive has been tested in a multicentre, multiphase trial by the World Health Organization. Results from this trial showed that an ester of testosterone, testosterone enanthate (TE), administered at a dose of 200 mg/week, has a very high contraceptive efficacy, and suggested that, at least in some populations, androgen alone might provide a viable option for the control of male fertility. It has been claimed that testosterone represents one of the gender-related risk factors for coronary artery disease (CAD) in men. Epidemiological and interventional studies have failed to establish a convincing relationship between testosterone and high density lipoprotein cholesterol (HDL-C). Therefore, there is concern about possible negative effects on lipoprotein asset of an androgen-alone male contraceptive. In this study we analysed the effects of long-term (12 months) administration of TE (200 mg/week) in normal healthy men. Blood samples (six men > 10 h fast = Group 1; 30 men > 4 h fast = Group 2) were drawn from 36 men, monthly before the beginning of the injections (control), every 3 months throughout the study period (treatment), and 1 month after stopping TE injections (recovery). Total cholesterol (chol), triglycerides, HDL-C and LDL-C levels were measured in these samples. Biochemical parameters were also monitored. TE administration induced a significant decrease (15-20%) in HDL-C levels that was of comparable magnitude in men from both groups (fasting and non-fasting) and occurred regardless of basal HDL-C levels. No statistically significant effect on other lipoproteins was detected. Considering all men together, HDL-C levels were decreased in 78% of the men by month 3, 83% by month 6, 94% by month 9 and 97% by month 12 of treatment. In all men the HDL-C decrease was reversible within 1 month of stopping TE administration. It is concluded that: (1) injection of 200 mg TE/week causes a 15-20% decrease in HDL-C in normal men with no effect on other lipoproteins, (2) the suppressive effect of TE is maintained throughout the 1-year-injection period, and a direct relationship between the duration of TE administration and the proportion of men showing decreased HDL-C levels, was observed. (3) The HDL-C decrease was reversible within 1 month of stopping TE administration. These data will be important in designing further studies on male contraception, and in interpreting the relationship between testosterone levels, HDL-C levels and potential cardiovascular risk.