The serum levels of sialyl stage-specific embryonic antigen (SSEA-1) in 67 patients with adenocarcinoma of the lung were studied to assess their values for diagnosis. A solid-phase immunoradiometric sandwich assay with an FH6 monoclonal antibody was used. Another 49 healthy adults, 52 patients with benign pulmonary diseases, and 142 lung cancer cases with cell types other than adenocarcinoma were evaluated for comparison. The mean (+/- S.D.) levels (U/mL) for adenocarcinoma of the lung, lung cancer other than adenocarcinoma, benign pulmonary diseases and normal subjects were 182.9 +/- 309.7, 53.5 +/- 22.6, 38.9 +/- 17.1 and 30.5 +/- 6.5, respectively. The mean serum sialyl SSEA-1 level was significantly higher in adenocarcinoma of the lung, compared with lung cancer other than adenocarcinoma (p < 0.001), benign pulmonary diseases (p < 0.001), or normal subjects (p < 0.001). For late stage (Stages III and IV, n = 58) adenocarcinoma of the lung, the mean (+/- S.D.) serum sialyl SSEA-1 level (204.3 +/- 327.6 U/mL) was significantly higher than that of earlier stage (Stages I and II, n = 9) adenocarcinoma of the lung (39.9 +/- 19.3), p < 0.001. There was no statistical difference among the mean serum levels of various histologic types of lung cancer other than adenocarcinoma (p > 0.05). In the lower range of values, considerable overlap existed between adenocarcinoma of the lung and lung cancer other than adenocarcinoma. However, very high sialyl SSEA-1 levels (> 140 U/mL) were only encountered in late stage adenocarcinoma of the lung (22/58).(ABSTRACT TRUNCATED AT 250 WORDS)