The predictive value of serial versus isolated measurements of transfer factor for carbon monoxide (TLCO) in the diagnosis of pneumocystis carinii pneumonia (PCP) in a cohort of 474 HIV-1 seropositive patients, with all stages of HIV disease, was evaluated. Two groups of patients were studied, one group with serial lung function measurements (Group 1) and another with only a single set of measurements (Group 2). During the study period 118 patients performing serial tests developed a respiratory illness of which 58 were performing monthly and 60 three monthly measurements of lung function (Group 1). In 36 patients from Group 1, where PCP was diagnosed, monthly lung function tests showed a decrease in TLCO from 68% (+/- 3.2) (SEM), (8 weeks prior to illness), to 44% (+/- 2.5) predicted normal at presentation, whereas in 22 patients who did not have PCP, TLCO fell from 71% (+/- 4.5) to 57% (+/- 3.1). TLCO was thus reduced to lower values in these with PCP than in those without PCP (p < 0.05). A fall of TLCO of 5% from initial values when used as predictive for presence of PCP had a sensitivity of 75% and a specificty of 28% (positive predictive value 56%; negative predictive value 48%). TLCO was < 70% predicted in 72/78 patients with PCP who performed only single lung function tests (Group 2), which gave a sensitivity of 92% and a specificity of 71% as a diagnostic test for PCP when compared with the cohort as a whole. The positive predictive value was 34%, negative predictive value was 98%.(ABSTRACT TRUNCATED AT 250 WORDS)