A case-control study was conducted to describe lung cancer risk in a cohort of New Mexico underground U miners. The subjects included 65 cases and 230 age-matched controls, most with exposures below 3.50 J h m-3 (1000 WLM). The risk for lung cancer was increased for all cumulative exposures to Rn progeny of 0.35 J h m-3 (100 WLM) or greater. The odds ratios were unchanged with control for cigarette smoking. With exclusion of subjects with exposures above 3.50 J h m-3 (1000 WLM), the estimated excess relative risk was 0.3% per mJ h m-3 (1.1% per WLM). The risk was greater for younger subjects and the data were consistent with a multiplicative interaction between cigarette smoking and exposure to Rn progeny.