Immunocompromised patients with influenza A were identified in Stockholm during two influenza seasons. The predominant subtypes were H3N2 during 1988–1989 and H1N1 during 1990–1991. The median age of the 25 patients was 43 years (range, 3–80 years). Twelve patients had received renal transplants and had ongoing immunosuppression. Seven patients had received bone marrow transplants between 2 days and 3 years before becoming infected with influenza virus A. Two patients were in an aplastic phase, and four had chronic graft-versus-host disease with ongoing immunosuppression. Six patients had hematologic malignancies. Two of the 25 patients had severe infections. One of these infections occurred in a bone marrow transplant recipient during an aplastic phase and was fatal; the other affected a patient who had received a renal transplant. One bone marrow transplant patient had mild but protracted infection. The remaining 22 patients had mild influenza A. We conclude that influenza A in immunocompromised patients occasionally causes severe complications but in most patients is mild and self-limiting.