Abstract Introduction: We previously found that the occurrence of congenital Zika syndrome was under-reported in Mexico. It was postulated that high dengue virus antibody levels found at the population-level in endemic countries might have contributed to the occurrence of the regional pandemic of Zika disease. A case series of suspected cases of congenital Zika syndrome in a maternity hospital in Tuxtla Gutierrez, Chiapas, Mexico was assembled to assess why they were not reported and to explore the hypothesis of dengue virus antibody-dependent enhancement of Zika disease. Methods: We used a quantitative approach to describe clinical and imaging records and used data from interviews of a total of 13 suspected cases of congenital Zika syndrome. We also quantitated dengue 1 and 2 antibodies using an 80% plaque reduction neutralization test of sera specimens obtained from the mothers of these 13 cases and compared them to those of a group of mothers who delivered normal newborns in the same hospital. Results: Only one of the suspected cases was laboratory-confirmed because appropriate specimens were not collected from the newborns as required by the case definition. We found 1) microcephaly, 2) hypoplasia/hypogeneses, thinning or absence of brain structures, 3) multiple birth defects, 4) calcifications, and cysts, 5) meningocele/encephalocele, and 6) hydrocephalus in 100 %, 76.9 %, 38.5 %, 38.5 %, 30.8 %, and 23.1 %, respectively of the case series. The cases clustered geographically, and 77 % occurred between May 2016 to March 2017 and recalled or were told by a doctor they had Zika fever. There was a four-fold increased risk of congenital Zika syndrome among those with dengue 1 antibody as compared to those with dengue 2 antibodies (odds ratio = 3.6; 95 % confidence interval: 0.7, 20.5), reaching only borderline statistical significance. Conclusions: We found in the largest maternal facility of the capital of the State of Chiapas, in Mexico, that only 7.7 % of suspected cases were confirmed, and that the rather complex requirement of cerebrospinal fluid specimens or serological specimens of newborns for suspected cases of congenital Zika syndrome used during the pandemic resulted in low sensitivity of the surveillance system. The finding of higher levels of dengue 1 than dengue 2 antibodies in cases than the referent population, requires further evaluation and may suggest a role for dengue antibody-dependent response in Zika disease.