The frequency of Kallmann syndrome (hypogonadotropic hypogonadism and anosmia, HHA) was estimated in patients presenting with hypogonadism and patients with anosmia. Of 791 hypogonadal males 19 had HHA. The frequency of HHA was about 1:25 (n = 8/189) in outpatients questioned about their sense of smell, about 1:50 (n = 11/579) in patients whose blood samples were sent to us for chromosome analysis, and about 1:30 (n = 19/605) in males with hypogonadism and 46,XY chromosomes. The relation of patients with HHA to those with Klinefelter syndrome was 1:10 (n = 19/186). From 24 patients presenting with anosmia we found 1 hitherto undiagnosed case of HHA. The mean age at diagnosis was 24.8 and 24.9 years in our cases and cases from literature, respectively. These data provide evidence that Kallmann syndrome is not infrequent and that most patients remain undiagnosed until the third decade of life. Earlier diagnosis is emphasized by questioning each hypogonadal patient about his sense of smell because therapeutic success seems to be age dependent.