Genetic classification of retinitis pigmentosa (RP) can be problematic, due to a large number of isolated cases, reduced penetrance, and considerable variation in expressivity. Another confounder is a high proportion of affected female carriers in X-linked RP. Based on the genetic definitions of five different authors, a reclassification experiment was conducted with 350 Danish families. Agreement existed about a small "nucleus" of familial cases. Most definitions favored autosomal dominant inheritance at the expense of X-linked. The experiment revealed that methodological differences to a large extent might explain the considerable variation among reported genetic frequencies of retinitis pigmentosa.