Cutaneous non-Hodgkin's lymphomas are rare in patients with HIV-1 infection and almost all of the cases reported are of T-cell lineage with histopathological features of mycosis fungoides or Sezary syndrome. We studied 2 cases of mycosis fungoides in HIV-1-positive patients who were intravenous drug abusers and were in stage II and IV C2 (CDC'86), respectively. The first patient (stage II) had multiple, erythematous and infiltrated large plaques on the abdomen, back, arms and legs, whereas the second patient (stage IV) had smaller erythematous, slightly scaly and infiltrated pruritic plaques on the trunk and limbs. Their CD4 lymphocyte counts were 634 and 250 cells/mm3, respectively. Biopsies showed features consistent with mycosis fungoides, with an epidermotropic pattern. The immunohistochemical study revealed a T-cell lineage of this atypical infiltrate. Both patients partially responded to topical steroid ointment, showing moderate improvement. Further biopsies performed 6 months later confirmed the prior diagnosis of mycosis fungoides. No tumour stage was observed during a 2-year follow-up. We conclude that mycosis fungoides is rare in HIV-positive patients, but must be included in the differential diagnosis of erythematous plaques in these patients. In suspected, but non-diagnostic cases of mycosis fungoides in HIV-positive patients, only a close clinical and histopathological follow-up can confirm the diagnosis.