Cellular immune competence was quantitated in 100 postoperative cancer patients considered clinically free of tumor and compared with that of 200 preoperative cancer patients and over 400 cancer-free persons. Immunity was assessed by in vitro phytohemagglutinin (PHA)-induced lymphocyte reactivity and in vivo dinitrochlorobenzene (DNCB) contact sensitivity. The immune reactivity of the cured cancer patients correlated with the histology of the tumor previously excised. Patients cured of squamous carcinomas had high incidences of impairment of lymphocyte reactivity and DNCB contact sensitivity similar to that found in preoperative patients. Patients with sarcomas, melanomas, and adenocarcinomas, however, had lymphocyte reactivity higher than both their preoperative counterparts and normal subjects, and did not display the abnormalities of DNCB contact sensitivity present in the comparable preoperative groups. A suppressant effect on lymphocyte reactivity of sera from preoperative patients with squamous carcinomas and sarcomas also occurred with sera from cured squamous carcinoma patients. The persisting high incidence of impaired cellular immunity in patients cured of squamous carcinomas indicates the need for investigation of genetic and environmental factors which may be responsible for the defects and determination of their relation to tumor induction. Treatment of these tumors may be improved by monitoring immune reactivity during therapy and by regimens that correct these defects. The intact cellular immune competence of patients clinically cured of sarcomas, melanomas, and adenocarcinomas indicates that patients treated for tumors of these histologic types who display abnormalities by the assays used may be sus- pected of having residual tumor.