Cryptococcosis is an epidemiological and immunological indicator due to the absence of Cryptococcus neoformans as a saprophyte in immunocompetent humans and the advantage of specific C. neoformans culture. On this basis, a report is presented on the CD4 lymphocyte count of 36 AIDS patients suffering from cryptococcosis and other concomitant or missing opportunistic AIDS-defining infections. In 26 out of 36 patients, i.e. 72%, a CD4 lymphocyte count of < or = 50/microL (mean value 39.5%) was found. Cryptococcosis as the sole opportunistic infection was diagnosed in 5 cases (13.9%). In 31 cases, various combinations of AIDS-associated diseases were found: Pneumocystis carinii pneumonia (PCP) (n = 19), cytomegalovirus infection (CMV) (n = 10), Kaposi's sarcoma (n = 6), Mycobacterium avium intracellulare infection (MAI) (n = 5), pneumonia (n = 2), toxoplasmosis (n = 2), Candida esophagitis (n = 1), tuberculosis (n = 1), lambliasis (n = 1), salmonellosis (n = 1) and wasting syndrome (n = 5). The conspicuous simultaneous occurrence or succession of pneumocystosis and cryptococcosis and the contrasting absence of aspergillosis and mucormycosis (zygomycosis) are commented. Based on the present observations in HIV-infected persons in Berlin, a CD4 lymphocyte count of < 150/microL may be used as a parameter indicating a predisposition for cryptococcosis as an airborne AIDS-defining infection. Attention is drawn to bird droppings as the sole habitat of C. neoformans and accidental niche of various other microorganisms.