Testosterone (T) has previously been shown to increase muscle size and strength in hypogonadal men [1, 2]. Although one of the main determinants of muscle force generating capacity is muscle architecture, changes thereof in response to T replacement therapy are largely unknown. This is despite the fact that muscle architectural parameters (pennation angle, fascicle length and muscle thickness) are known to change with disuse atrophy, sarcopenia and hypertrophy [3, 4]. Hence the aim of this study was to investigate whether T replacement therapy could mitigate muscle wasting in hypogonadal men and whether changes in muscle mass were associated with alterations in muscle structure. Thirty intermediate-frail and frail [5] men (72.6 ± 5.7 years) with total T ≤ 12nmol/L or free T ≤ 250 pmol/L received either a transdermal 1% T gel (n=16) at physiological doses (50 mg/day) or a matched placebo gel (n=14), for six months. The dose was adjusted according to serum T at day 10 and 3 months to maintain total T levels within the target range (18-30 nmol/L). Gastrocnemius medialis muscle architecture was assessed by ultrasound at baseline and after treatment. An analysis of covariance (ANCOVA) model adjusting for baseline frailty status was used to assess the effects of T treatment (compared to placebo) on muscle architecture, and only those participants who provided data at baseline and 6 months were included in the analyses. Significance was set at p≤0.05. After six months, there was a significant positive effect of T on muscle thickness compared with the placebo group (effect size 1.4 mm [95% confidence interval 0.3 to 2.5 mm, p=0.015]), increasing by 2.6% in the T group and decreasing by 5.4% in the placebo group. Pennation angle was unchanged throughout the study in both groups (effect size 1.2° [95% confidence interval -1.3 to 3.7°, p=0.32]). An age-associated decrease of 0.8% in the T group and 6.7% in the placebo group was detected in fascicle length though this was not alleviated by T treatment (effect size 1.9 mm [95% confidence interval -1.2 to 5.0 mm, p=0.22]). This study indicates that T replacement therapy helps to maintain lean body mass by improving muscle thickness in intermediate-frail and frail elderly men, confirming the role of T as an important regulator of muscle mass [1]. The lack of changes in pennation angle suggests that while T acted to prevent sarcopenia, it did not promote a sufficiently large accumulation of new contractile material along the tendon aponeurosis to increase pennation angle. It is not surprising that fascicle length decreased in this study as this is an effect of ageing [3] and usually would only increase in response to stretch of the muscle, i.e. with resistance training. Therefore, T therapy in elderly men with low T levels seems effective in promoting changes in muscle mass from a condition of atrophy to a state of normotrophy (involving little reorganisation of fascicle geometry) but not of hypertrophy, which is normally associated with an increase in pennation angle.