The volume doubling time (T2) of 52 lung metastases in 21 patients was calculated from measurements done on plain chest radiographs. Follow-up times ranged from 14 to 819 days. The measurements were fairly well reproducible in the majority of patients, although considerable discrepancies in T2 estimates made by two independent observers were found in a few patients. The median doubling time was 34.9 days (estimated 95% range 3.9 to 352 days). The variation of T2:s between patients was significantly (P = 0.0001) larger than that between T2: of multiple metastases in the same patients. The growth of the metastases seemed to be well described by a simple exponential function in all patients with more than two measurements, without evidence of Gompertzian growth. There seemed to be a linear correlation between the logarithm of T2 and log-survival time from diagnosis of metastatic disease, even if only one third of the variation of survival times between patients could be explained by differences in T2. T2 was not a significant factor for survival in Cox-analysis (P = 0.10).