In a blinded, randomized design, six histopathologists with an interest in gynecological pathology examined the inter- and intraobserver variation of the histopathological diagnosis of endometrial hyperplasia according to the World Health Organization (WHO) classification of 1975 and the new WHO classification of 1994. On four occasions, the pathologists assessed hematoxylin/eosin-stained slides from 128 cases originally diagnosed and coded in the Snomed system as endometrial hyperplasia. In the first and third rounds, the slides were classified according to the 1975 classification and in the second and fourth rounds according to the 1994 classification. The overall interobserver agreement in the two rounds where the 1975 classification was used was 0.47 and 0.51, and the kappa values 0.24 [95% confidence interval (CI) 0.21-0.27] and 0.30 (95% CI 0.27-0.33). The overall interobserver agreement in the two rounds using the 1994 classification was 0.45 and 0.41 and the kappa values 0.25 (95% CI 0.23-0.28) and 0.20 (95% CI 0.17-0.22). Reducing the classification to two categories with clinical significance (atypical endometrial hyperplasia versus others in the 1975 classification, and atypical endometrial hyperplasia, complex versus others in the 1994 classification) increased the overall agreement of the 1975 classification in both rounds to 0.91 and of the 1994 classification to 0.92 and 0.90. The kappa values increased to 0.54 (95% CI 0.49-0.58) and 0.49 (95% CI 0.45-0.54) in the 1975 classification and to 0.59 (95% CI 0.54-0.63) and 0.42 (95% CI 0.37-0.46) in the 1994 classification. The intraobserver overall agreement for the 1975 classification ranged from 0.80 to 0.55 and the kappa values from 0.70 (95% CI 0.58-0.81) to 0.28 (95% CI 0.17-0.39). The intraobserver overall agreement for the 1994 classification ranged from 0.71 to 0.46 and the kappa values from 0.60 (95% CI 0.51-0.70) to 0.20 (95% CI 0.09-0.30). It is concluded that there is considerable inter- and intraobserver variation using both the 1975 and the 1994 classifications of endometrial hyperplasia. We propose that there is need for a specification and for a simplification of the classification.