Groups of 6-wk-old male BALB/cByJ mice were injected intratracheally (IT) with viable conidia from Blastomyces dermatitidis strains (FW or CR) harvested from 2-wk-old cultures (Pine-Drouhet agar at 30°C) and separated from hyphal elements by polycarbonate filtration (5 μm). Inocula (0.05 ml) were 93 to 97% conidia in nonpyrogenic normal saline (NPNS) with > 90% viability. Quantitative cultures of the lungs and trachea of mice killed immediately after injection of 104 conidia (FW) confirmed that the inoculum was evenly distributed between the lungs, with virtually no conidia retained in the trachea. Animals were observed for 354 days for weight change, mortality, extrapulmonary dissemination and histopathologic changes. Mice inoculated with 104 FW began dying on Day 44, with a median survival of 92 days. A decrease in mean weight compared with that in control mice was noted by Day 55. In contrast, mice inoculated with 104 CR or NPNS neither died with blastomycosis nor lost weight in 329 days. Variation of the FW inoculum by tenfold increments produced comparable dose-dependent alterations in both mortality rate and weight change curves. Fifty percent survival was 32, 36, 92, or 172 days for mice inoculated with 106, 105, 104, or 103 conidia, respectively. Dissemination from the lungs to the liver, spleen, kidney, testis, and brain was found in the cachetic mice that were killed. Pathologic features of pulmonary and disseminated infections were comparable to those in human disease. This quantitative, reproducible model of chronic murine blastomycosis mimics the human disease in many respects and should provide a basis for future studies of the immunology, pathogenesis, and therapy of chronic blastomycosis.