European vipers are typical capital breeders and most studies have revealed that females usually reproduce once every two or three years and that, in some cases, females reproduce once during their lifetime. This paper presents evidence, based on monitoring marked snakes in consecutive years, that several female adders (Vipera berus) from a montane environment from the Northern Romanian Carpathians reproduced annually (2007–2008), and also gives preliminary data on other female reproductive traits. Reproductive females and their offspring were smaller and lighter than V. berus individuals from most previously studied populations. Maternal traits were not significantly correlated with litter and offspring characteristics.