It has been thought that squamous severe atypical metaplasia of the bronchus is reliably precancerous. The canine subcutaneous bronchial autograft model for studying the progression of epidermoid carcinogenesis (normal----regular squamous metaplasia----mild, moderate, and severe atypical metaplasia----squamous cell carcinoma) provides evidence that severe atypical metaplasia of the bronchial epithelium is reversible. Among 148 subcutaneous bronchial autografts that had serial sampling of the epithelium and exposure to implants of methylcholanthrene, severe atypical metaplasia was noted in 28 that received only a single implant. During the total carcinogen exposure (median 24.5 months), 9 of 28 (32%) developed squamous cell cancer, and 19 of 28 (68%) regressed toward normal. Severe atypical metaplasia was noted in 34 subcutaneous bronchial autografts that received two or more carcinogen implants: epidermoid cancer developed in 26 of 34 (76.5%), and regression toward or to normal occurred in 8 of 34 (23.5%). Severe atypical metaplasia was not detected in 53 subcutaneous bronchial autografts: 19 that received only a single implant and 34 that received two or more implants. Progression and regression occurred among these subcutaneous bronchial autografts in proportions similar to those found in subcutaneous bronchial autografts wherein severe atypical metaplasia was seen. Among 33 subcutaneous bronchial autografts initially studied after 6 months of exposure to carcinogen, progression to severe atypical metaplasia was seen 3 months later in 19 of 33 that had additional exposure; in the same interval regression of epithelial abnormalities occurred in 14 of 33 subcutaneous bronchial autografts that had no additional exposure (p less than 0.05). We have presented evidence that severe atypical metaplasia includes at least three cell populations: one committed to cancer without further stimulus, one that regresses despite further carcinogen exposure, and one that requires additional carcinogen to progress to cancer. At least in this model, severe atypical metaplasia is not inexorably precancerous. The subcutaneous bronchial autograft model is suitable for seeking biologic indicators of irreversibility.