Context: Evidence on the long-term and general health of males with gynecomastia is entirely lacking.

Objectives: To assess health prior to and following a diagnosis of gynecomastia.

Methods: A register-based cohort study of 140,574 males, of which 23,429 males were diagnosed with incident gynecomastia and age- and calendar-matched (1:5) to 117,145 males without gynecomastia from the background population. Males with gynecomastia were stratified into males without (idiopathic) or with a known pre-existing risk factor (disease/medication). Cox and logistic regression models investigated associations of disease risk according to ICD-10 chapters following and prior to gynecomastia diagnosis.

Results: A total of 16,253 (69.4%) males in the cohort were identified with idiopathic gynecomastia. These males had a statistically significant higher risk of future disease across all included disease chapters (HR range: 1.19 to 1.89), with endocrine diseases representing the greatest disease risk (HR 1.89, 95% CI: 1.76 to 2.03). The highest sub-chapter disease risk was observed for disorders of endocrine glands (OR 7.27, 95% CI: 6.19 to 8.54). Similarly, the odds ratios of comorbidities were higher across all included disease chapters (OR range 1.05 to 1.51), except for psychiatric disease (OR 0.72, 95% CI: 0.68 to 0.78), with the highest association with musculoskeletal/connective tissue (OR 1.51, 95% CI: 1.46 to 1.57) and circulatory (OR 1.36, 95% CI: 1.29 to 1.43) diseases.

Conclusions: The presence of idiopathic gynecomastia is an important first clinical symptom of an underlying disease and a significant predictor of future disease risk. These findings should stimulate more awareness among healthcare providers to increase identification of gynecomastia and its causes in males.