Introduction: HDL cholesterol (HDLc) has an important role in atherosclerosis, with levels <40 mg/dl in men and <48mg/dl in women recognised as a cardiovascular risk factor. Nowadays there are many treatments available to LDL cholesterol, but HDLc management has a minor role in secondary prevention. Although statins and fibrates may increase its levels, lifestyle intervention and smoking cessation are the main therapeutic tools. Methods: retrospective analysis of patients that completed a cardiac rehabilitation program (CRP) after an acute coronary syndrome (ACS) between 2006 and 2016. We compared laboratory findings prior and after CRP. The aim was to analyse the profile of the patients with low HDL, their level evolution and the factors that could explain very low levels of HDL (<30 mg/dl) after CRP. Results: we included 1250 patients, of which 18.7% had very low levels of HDLc. These were predominantly male (94.7%, p<0.01), had a higher proportion of diabetes mellitus (35.1%, p<0.01) and overweigh (BMI>25 79.4%, abdominal perimeter 102.1 ± 11.7, p<0.01) and also had lower levels of LDLc, and higher values of HbA1c (6.4 ± 1,5, p<0.01). After completing CRP, 517 patients (37.2%) had a higher HDL levels, and this improvement was more frequent in women (18.2%, p = 0.03) and in those who stopped smoking (60.7%, p <0.01). Conclusion: the role of HDLc as a cardiovascular risk factor is probably underestimated, since it is very frequent in patients with ACS. Currently its treatment is based on lifestyle intervention, but in many cases HDLc does not improve despite CRP. The question now is whether low HDL must be contemplated only as a marker of risk, or if it should be considered as a therapeutic target.