OBJECTIVES: Financial protection of the affiliated workers to the Social Security Mexican Institute (IMSS) is one of its core functions; however it also generates outlays to the health system. This study estimates the associated cost to additional sick leave days due to medical causes and/or due to inefficiencies on the system. METHODS:  A cost study was made from the perspective of the institution through a prospective cohort of male and female patients to whom it was prescribed temporal sick leave days due to diseases not related to the work environment (general disease), a follow-up was scheduled until their return to their work place and the cost of medical attention was calculated; this included ambulatory consult, drugs, lab exams, urgency service assistance, surgery, hospital time and payment under the concept of sick leave. All patients must have an estimation of the probable number of recovery days, those who required additional days were identified and the causes that triggered a longer lapse. The additional costs for additional sick leave days were calculated. Significant statistical differences were established in the cost through a Mann Witney’s U test. RESULTS: A total of 1019 patients were included with an average age of 37±11 years, 49% of which were men. These patients were prescribed with 12,273 sick leave days, the average per patient was 12 days (4-93 days). The total cost of the attention of this patients was of US$ 781,806.76. 17.9% of the patients had additional sick leave days. The average cost of the medical attention of the patients without additional days was of US$773.74± 1135.01 and of the patients with additional days was of US$737.74 ± 960.38, the differences were statistically significant (p<0.005). CONCLUSIONS:The additional sick leave days have not a significant impact in the attention cost of the patients affiliated to the IMSS