T and B peripheral blood lymphocytes were evaluated in 17 patients with clinical and histological evidence of mycosis fungoides and in ten patients with benign lymphoplasias of the skin. The T subpopulation (T-Ea) characterized by a rapid rosette formation with sheep erythrocytes was also measured in these patients. While both T-total population (T-Et) and T-Ea subpopulation were within the normal range in pseudolymphoma group, a statistically significant decrease was observed in T-Ea and T-Et populations in the mycosis fungoides group. Moreover, the decrease in T-Ea subpopulation was found in all cases under investigation, even when T-Et population was still at the normal levels. A close relation between T-Ea values and clinical status was observed. B and null cells were substantially unchanged in all patients.