A number of published papers have dealt with the comparison of birth prevalences of congenital cardiovascular malformation (CCVMs). The feasibility of meaningful intercountry comparison was explored during the visiting fellowships to Dr Andrew Czeizel of Charlotte Ferencz and Dr Francine Lys. Data from three Hungarian studies, one USA and one Belgian study are presented here for selected CCVMs. Differential perinatal and diagnostic circumstances which lead to possible causes of bias involve four main domains: study population, ascertainment of cases, categorization of CCVMs and diagnostic definitions. Unique features are highlighted as a descriptive framework which will promote the comparability of epidemiologic data from various regions.