This study investigated the safety of administration of long-acting parenteral testosterone undecanoate (TU) to 122 hypogonadal, mainly elderly men, aged 59.6 ± 8.0 years (range 18-83 years old), with baseline testosterone levels between 5.8 and 12.1 nmol l(-1) (mean ± SD = 9.3 ± 1.7). Patients were followed for 24 months. Plasma testosterone rose from 9.3 ± 1.7 to 14.9 ± 4.5 nmol l(-1) (P< 0.01) at 3 months, then stabilised at 19.2 ± 4.6 nmol l(-1) after 6 months. International Prostate Symptoms Scores and Residual Bladder Volumes decreased significantly (P < 0.001) over the 24 months while the haematocrit increased significantly (P < 0.001) during the first 15 months and then levelled off. Statistical analysis with expressing values as means ± SD masks excesses above reference values of individual patients. These excesses were noted in low numbers, were permanently present in some but not in other individuals, and did not increase in number over the 24 month study period. Over 24 months treatment with TU appeared acceptably safe, but longer and larger scale studies are needed.