High rates of lost to follow-up (LTFU) in the Prevention of Mother-To-Child Transmission of HIV (PMTCT) programs in Cameroon will only contribute in hindering the successful implementation of the program. The objective of this study was to determine the reasons of LTFU of mother-child pairs enrolled in the PMTCT program in Dschang District Hospital (DDH): Cameroon. Methods: A retrospective cohort study was carried out in HIV+ exposed children delivered in the DDH from1st Jan 2012 - 31st Dec 2014, who were greater than or equal to 18 months at the study period (1st August - 30th September 2016) and whose mothers were enrolled in the PMTCT program for at least 3 months. Children were considered LTFU if they did not return to the hospital for the establishment of their HIV status at 18 months. A complementary cross-sectional study was done whereby a structured questionnaire was administered to the LTFU group via telephone calls, in order to determine the causes of LTFU. Data was collected from hospital registers and analyses done using Epi info 7.1.3.3 software. Results: A total of 141 mother-child pairs were eligible for the study, 76 were reachable via phone calls and 36 (47.37%) met the case definition of LTFU. Out of the 36 (47.37%) children LTFU, lack of information, 19(65.52%); lack of support from male partners, 4(11.11%); Poor behavior of health personnel, 2 (5.56%); forgetfulness, negligence/time wasting, unavailability of results, fear of child being infected, and family problems were all listed at equal proportions of, 3(6.90%) as well as financial problem, 1 (2.78%) were all causes of LTFU identified in this study.