1. A study of 150 middle-aged male industrial employees has shown significant positive correlations between plasma levels of high-density-lipoprotein (HDL) cholesterol and both serum testosterone and alcohol intake, and significant negative correlations between HDL cholesterol and both serum thyroxine and obesity. These associations persist when examined by multiple linear regression, indicating their independence. 2. Significant positive correlations are also shown between plasma triglyceride levels and both obesity and serum thyrotropic hormone (TSH) levels. 3. There are no evident relationships between serum oestrone or oestradiol and either HDL cholesterol or triglyceride levels, nor between any of the hormones and either total or low-density-lipoprotein (LDL) cholesterol. 4. Because of the potential importance in relation to coronary heart disease prevention, further studies are needed to try and understand the mechanisms of the associations between HDL cholesterol and obesity, alcohol intake and thyroid and sex hormone levels.