Dermatitis herpetiformis (DH) is an extra-intestinal manifestation of coeliac disease. The highest currently reported prevalence of DH is in Finland, but knowledge of diagnostic delay is limited. This study investigated the duration of rash prior to diagnosis in 446 patients with DH, analysing the results in 3 periods of 15 years. The diagnosis was considered delayed when the duration of rash before diagnosis was 2 years or longer. Factors associated with delayed diagnosis were analysed. Within the 45 years, the median duration of rash before diagnosis decreased significantly, from 12.0 to 8.0 months (p?=?0.002) and the occurrence of a delayed diagnosis decreased from 47% to 25% (p?=?0.002). Female sex, the presence of villous atrophy, and a diagnosis of DH before the year 2000 were significantly associated with delayed diagnosis. In conclusion, the present study showed that one-quarter of patients currently have a diagnostic delay of 2 years or more, which is far from ideal.