A case-control study was performed in Libreville, Gabon, to determine whether a relationship can be established between the relatively high proportion of non-Hodgkin lymphoma (NHL) among all cancer cases and the high seroprevalence rate of HTLV-I observed, and to discover whether cases of adult T-leukemia/lymphoma (ATLL) related to HTLV-I exist in Gabon. From November 1987 to April 1989, a total of 32 patients with NHL were recruited; 6 were infants with Burkitt's lymphoma and 26 were adults with NHL. Each patient was matched with 2 asymptomatic controls for age, sex and ethnic group. HTLV-I serology was done by ELISA and Western blot. Comparison of the groups was done by chi-square analysis. None of the 6 infants with Burkitt's lymphoma and none of their controls had antibodies to HTLV-I. Of the 26 patients with NHL, 7 (26.9%) had HTLV-I antibodies. Among the 52 controls, the HTLV-I rate was 13.4% (n = 7). There was no difference between cases and controls (Fisher's exact test, p = 0.16). Among the 26 NHL, 4 cases fitted the criteria of ATLL and were HTLV-I-positive; 3 others who were positive for HTLV-I were a woman with lymphoblastic gastric NHL and 2 old men with an unclassified lymphoma. From the results of this limited series it is not possible to state that there is an association between NHL and HTLV-I infection. Nevertheless, cases of ATLL related to HTLV-I are reported from this area. Based on the HTLV-I seroprevalence rates reported in Gabon, the estimated incidence rate of ATL among seropositive people in Gabon appears much lower than in Japan. Different explanations can be proposed, but under-diagnosis of ATLL is probably one of the main factors.