It is still under discussion whether subclinical hypothyroidism is a biochemical syndrome or a disease associated with an increased risk for development of vascular diseases due to lipid elevation. Therefore, we investigated lipid values in 40 patients with subclinical hypothyroidism, which is defined in terms of normal (N =26) or slightly increased (N= 14) basal TSH values and/or an exaggerated TSH response (N=34) to TRH (>25 mU/l). Patients with increased lipid values were treated with L-thyroxine and reanalysed three months later. Mean levels of total cholesterol, LDL- and HDL-cholesterol and triglycerides in patients with subclinical hypothyroidism were comparable with those in normal subjects. Individual analysis, however, revealed hyperlipoproteinaemia (HL) in 22.5% of the patients investigated (HL type Ila in seven, type IV in two patients). Thyroid function was the same in affected patients as in those with normal lipid values, whereas higher age was significantly more often associated with this syndrome (p <0.01). Treatment with L-thyroxine resulted in a significant decrease in total and LDL-cholesterol (p<0.05), although a normalization of their lipid values could be obtained only in half of the patients. None of the subjects with hyperlipoproteinaemia had a history or clinical signs of actual vascular disease. Although the incidence of hyperlipoproteinaemia in our study group of patients with mild subclinical hypothyroidism (22.5%) is comparable to that of the normal population (21.5%), it is more severe in the former group (LDL-cholesterol in patients 5.26±0.58 vs 4.8±0.56 mmol/l in controls: p<0.05). Formally, approximately half of the patients with this syndrome benefit from L-thyroxine treatment with a normalization of their total and LDL-cholesterol. In the remaining patients with hyperlipoproteinaemia, elevated lipid levels do not seem to be associated with borderline thyroid dysfunction. We conclude that indiscriminate L-thyroxine treatment is not justified in subclinical hypothyroidism.