Context. Though it is known that serum testosterone (T) concentrations are related to libido, the strength of that relationship in community-dwelling men has not yet been determined.

Objective. To assess the strength and significance of the association between aging men's self-reports of libido and serum T concentrations.

Design. A community-based evaluation of men's health and aging. Three data collection waves: baseline (T1: 1987-89) and follow-ups (T2: 1995-97, T3: 2002-04). Libido was measured on a 14-point scale assessing self-reported frequency of desire and thoughts/fantasies; "low libido" was defined as a score of less than 7/14.

Setting. An epidemiologic study in greater Boston, MA, USA.

Participants. 1632 men aged 40-70 y at baseline, with follow-up on 922 (56%) at nine years (T2) and 623 (38%) at 15 yr (T3).

Main outcome measures. Total and calculated bioavailable testosterone (TT and BT) .

Results. Three hundred eighteen (19%) subjects reported low libido at baseline. Libido and T displayed a significant association. However, the difference in mean T levels between those subjects with low libido and those without was small; analyses indicated a 3.4 ng/dL (.12 nmol/L) increase in TT per unit increase in libido. Subjects reporting low libido exhibited an increased but modest probability of exhibiting low T. Dividing T concentrations by the number of androgen receptor gene cytosine, adenine, guanine repeats did not enhance associations.

Conclusions. Libido and T concentrations are strongly related at the population level. However, the value of individual patient reports of reduced libido as indicators of low T levels is open to question.