Myotonic dystrophy type 1 (DM1) is characterized by both a premature appearance of age-related phenotypes and multiple organ involvement, which affects skeletal and smooth muscle as well as the eye, heart, central nervous system, and endocrine system. Although erectile dysfunction (ED) is a frequent complaint in patients with DM1, it has not been investigated in great depth. Hypogonadism, which is reported to be one of the physical causes of ED in the general population, frequently occurs in DM1. We planned this case-control study to evaluate the relationship between hypogonadism, as defined by the sexual hormone profile (FSH, LH, testosterone (T) and prolactin) and ED, as assessed by means of an internationally validated self-administered questionnaire (IIEF). DM1 patients had significantly increased mean levels of both gonadotropins (FSH and LH) (p < 0.0001) and a reduced mean level of T (p < 0.0001) when compared to controls. Twelve patients were eugonadic (normal LH, T, and FSH), while 18 displayed hormonal evidence of hypogonadism, characterized by tubular failure (increased FSH) in all the subjects and associated with interstitial failure in 14 subjects: seven with primary hypogonadism (increased LH and reduced T) and seven with compensated hypogonadism (increased LH and normal T). Patients with hormonal evidence of interstitial failure had a larger CTG expansion (p = 0.008), longer disease duration (p = 0.013), higher grade of disease (p = 0.004) and lower erectile function score (p = 0.02) than eugonadic patients. Impotence occurred in 13/14 hypogonadic patients with interstitial failure and in 5/12 eugonadic patients (p = 0.017, OR = 18.2).