Hamsters of three types designated as inbred cream (Epp/e/e), linebred white (EPP/cdcd/RB/A), and linebred albino (EPP/cdcde/e) were thoroughly examined histopathologically for spontaneous diseases. All hamsters were maintained simultaneously for life under identical standard laboratory conditions. Marked differences were found in longevity of the animals and in incidence, sites, patterns, and types of spontaneous diseases. In cream hamsters (CH), survival time was shorter than in white hamsters (WH) and albino hamsters (AH). More tumors and malignant lesions unrelated to survival were found in AH compared to CH and WH; also, the multiplicity of neoplasms were more pronounced in AH. The predominating tumor types differed in each line: Pancreatic islet cell neoplasms were most common in CH, adrenal gland tumors predominated in WH, and thyroid gland tumors in AH. Also, the relative incidence of spontaneous tumors varied among the lines. Some tumors seemed strain-specific and were not seen in other lines; malignant melanomas, for example, occurred only in CH and WH. Certain neoplasms, e.g., those of the thyroid and adrenal glands, were found more often in one sex than the other. The three hamster groups differed also in nonneoplastic diseases. Detailed histopathologic findings are presented and compared with data on the Syrian golden hamster, the ancestral line of these three groups.