T cell immune function was further evaluated in cystic fibrosis (CF) patients. CF patients, regardless of severity of pulmonary disease or colonization with Pseudomonas aeruginosa, had lower percentages of T helper cells (p less than 0.01) and decreased T helper function as demonstrated by diminished T help for control B cell pokeweed mitogen-stimulated IgG (p less than 0.01) and IgM (p less than 0.01) synthesis. Increased T suppressor function, measured by co-culture with control B and T cells, was noted in only 25% of CF patients. CF patients' T cells exhibited decreased allogeneic T cell cytotoxicity compared to normal controls (p less than 0.01). This study extends previous studies demonstrating decreased T cell functions in CF patients and indicates a specific decrease in T helper function.