In order to determine the diagnostic and prognostic significance of low pleural glucose and pH levels, we executed a prospective study of these parameters and cytologic yield in 77 cases of malignant pleural effusions diagnosed from 116 consecutive thoracoscopies. The extension of the neoplasms detected by thoracoscopy as well as the results of our attempts to carry out pleurodesis by talc were also studied. Pleural glucose levels were less than 60 mg/dl in 16 cases, and the cytologic yield was positive in 14 of these cases (87%). The glucose value rose above this level in 61 cases, and the cytology was positive in 30 cases (49%; p less than 0.006). A pH less than 7.30 was encountered in 18 cases and there were positive cytologic findings in 14 of these cases (78%). The pH was above 7.30 in 46 cases, and cytologic studies were positive in 22 cases (48%; p less than 0.03). There were ten cases in which both the glucose and pH levels were low, and the cytology was positive in 9 cases (90%), while there were 40 cases in which both the glucose and pH levels were high and cytologic yields of 20 of these cases were positive (50%; p less than 0.03). The extension of the lesions observed during thoracoscopy showed important differences as far as this related to the glucose levels (p less than 0.005), but this relationship as it concerned pH levels was even more significant (p less than 0.0002). The differences were also highly significant (p less than 0.003) when the glucose and pH were jointly considered.(ABSTRACT TRUNCATED AT 250 WORDS)