A 39-year-old male with follicular non-Hodgkin's lymphoma was repeatedly studied with respect to DNA rearrangements with the two probes pFL-1 and pFL-2, representing two segments of chromosome 18. The oncogene BCL2, detected by pFL-1, was as expected translocated to the J region of the immunoglobulin locus. The standard BCL2 translocation was found in three samples, one obtained at diagnosis, one ten months later, and one after 5 years. Another translocation was found with the probe pFL-2 hybridizing with a region located about 20 kb 3' from BCL2. This latter rearrangement was found only in the first biopsy, which was obtained at the time of diagnosis, but not in the two later samples, when the morphology of the lymphoma was unchanged. No cytotoxic therapy had been given in the interval from diagnosis to disappearance of the latter rearrangement. Thus, one of the observed translocations (pFL-2) was detected only in the first biopsy, while the other (pFL-1) was a clonal marker in all three biopsies. The finding suggests that clonal evolution does not necessarily mean clinical progression.