Tumours occurring in 900 male and 900 female mice of the strain NMRI (BOR: NMRI (SPF Han), breeder: Winkelmann, Borchen) employed as control groups in 18 different studies lasting for 21 months on average (study No. 8 for 20 months, study No. 5 for 24 months) were compiled on the basis of histopathology reports. 883 male and 874 female mice were evaluated. The remaining animals were unavailable due to autolysis. A total of 1522 tumours (689 in males, 833 in females) were seen in 1061 mice (493 males, 568 females), 830 of which were classified as malignant (318 in males, 512 in females). About 70% of all primary tumours were located in the haemolymophoreticular tissue, lungs, ovaries, adrenals and uterus. Average incidences of primary tumours found in the following organs were: haemolymphoreticular tissue 22.9%, lungs 21.6%, ovaries 12.1%, adrenals 6.9%, uterus 6.6%, liver 4.4%, Harderian gland 4.4%, pituitary 3.5%, mamma 3.3% and various other organs with a tumour frequency below 3%. Male mice exhibited markedly more tumours of the lungs, adrenals, liver and Harderian glands, while tumours of the haemolymphoreticular tissue and pituitary were predominant in females. A comparison of these data with those from preceding two-year or life-span studies using NMRI mice from other breeders revealed a certain increase in primary tumours of the adrenals, Harderian glands as well as pituitary. This result could be due to more frequent examinations.