Adducted thumbs are an uncommon congenital malformation. It can be an important clinical clue in genetic syndromes, e.g. the L1 syndrome.A retrospective survey was performed including patients with adducted thumbs referred to the Department of Clinical Genetics between 1985 and 2011 by perinatologists, (child) neurologists or paediatricians, in order to evaluate current knowledge on the genetic etiology of adducted thumbs. Twenty-five patients were included in this survey. Additional features were observed in 88% (22/25). In 25% (4/16) of the patients with adducted thumbs and congenital hydrocephalus L1CAM gene mutations were identified. One patient had a mosaic 5p13 duplication. Recommendations are made concerning the evaluation and genetic workup of patients with adducted thumbs.