The potential role of the hormone testosterone in the risk for myocardial infarction is investigated in this study of middle-aged men and women compared with a large random control sample from the general population. Radioimmunoassay was used to measure testosterone levels in hair, approximately 1 month and 3 months before an ST-elevation or non-ST-elevation acute myocardial infarction. Mean testosterone levels were measured for middle-aged men and women (n = 168) with diagnosed myocardial infarction (the acute myocardial infarction [AMI] cases). As controls, n = 3,150 randomly selected subjects from the general population of similar age were measured at 1 time point. No significant difference in testosterone levels in hair was found 3 months before AMI for men and women compared with the controls. However, 1 month before AMI, the testosterone levels were decreasing (p < 0.001) for both men (from 2.84 to 2.10 pg/mg) and women (from 1.43 to 1.10 pg/mg), indicating that a decrease in testosterone concentrations precedes a severe cardiac event. Conventional cardiovascular risk factors were tested as confounders but did not alter this tendency. The AMI cases were also compared with a randomly selected second control group from the general population (n = 205), for whom comparable segmental hair analyses were conducted. A tendency of some decreasing testosterone levels, also in the small control group, was only significant for men. This control group was a small sample, and there might be some natural biologic variation in testosterone levels over time. This study indicates that decreased testosterone levels may be among the pathophysiological processes preceding myocardial infarction and merits further investigation. (c) 2022 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)