This report describes a case of lymphoid interstitial pneumonia (LIP) associated with polyclonal gammopathy and selective depression of cellular immunity. Enhanced humoral immunity is suggested because the patient exhibited increased levels of circulating immunoglobulin (chiefly IgG) and a positive test reaction for rheumatoid factor. When first examined, the patient displayed depressed cellular immunity in that the results of his skin test were negative to dinitrochlorobenzene and phytohemagglutinin (PHA), his peripheral lymphocytes responded suboptimally to PHA, and he had decreased numbers of circulating T cells. When examined 6 yr later, the patient was again hypergammaglobulinemic, but he had regained some of his cellular immune function. Eight years after the patient was initially seen, he showed further improvement in cellular immune function. Analysis of bronchoalveolar lavage cells showed that the patient had a normal proportion of OKT4+ (inducer/helper) cells, but a reduced number of OKT8+ (suppressor/cytotoxic) cells. Thus, his improvement may have been associated with a correction of an imbalance of immunoregulatory T cells. Furthermore, because the disease was active on initial examination but less active after the second evaluation, malfunction of his cellular immune functions may have been involved in the pathogenesis of LIP.