The clinical significance of nontuberculous mycobacterial isolates and presentation of mycobacteriosis was compared in HIV-negative patients with or without preceding immunosuppression. Patients with nontuberculous mycobacterial isolates (n = 139), mainly from the respiratory system, were divided into three groups: those who had had previous immunosuppressive treatment (24%), those with other underlying diseases (54%) and those without predisposing factors (22%). The distribution of mycobacterial species among the various patient groups was similar. The immunosuppressed patients fulfilled the criteria of the American Thoracic Society for clinical mycobacteriosis less frequently (18%) than those with other underlying diseases (32%) or without predisposing factors (45%), p = 0.07, the difference being more striking for patients with Mycobacterium avium complex isolates. This was partly due to the difficulty in distinguishing the relevant symptoms from those caused by the underlying disease. The proportion of patients receiving antimycobacterial therapy differed similarly (18%, 21%, 45%, respectively). Among the immunosuppressed patients, positive acid-fast smears were significantly less common and polymicrobial infections, initial lymphocytopenia, fever and fatal outcome significantly more common. About half of the immunosuppressed patients died within one year. In order to better define patients requiring treatment, the criteria for localized mycobacteriosis among immunosuppressed patients should be reevaluated.