From April 1977 through March 1978, 28 presumptive cases of Legionnaires' disease were identified among 432 consecutive candidates having paired sera or tissue samples submitted to the Massachusetts Public Health Laboratories. Among the subgroup of 209 candidates with documented diffuse pneumonia and temperature of 39°C or above, 24 (11.5%) had Legionnaires' disease whereas the diagnostic yield was only four of 223 (1.8%) among the remainder. The case-fatality rate was two of 28 (7%). Patients with Legionnaires' disease when compared to the entire group of candidates were similar in mean age (49 versus 48 years) and frequency of immunosuppressant therapy (15% versus 12%) but were more often male (64% versus 47%) with underlying chronic illness (46% versus 22%). Complement fixation tests against Mycoplasma pneumoniae (whole organisms) showed seroreactivity in 81% of Legionnaires' disease (LD) cases (22 of 27) compared to 13% of non-LD cases; conversely, 29% of all cases seropositive for M. pneumoniae (22 of 75) were seropositive for the LD bacterium compared to only 1% (five of 357) of the remainder. The coincidence of seroreactivity for M. pneumoniae and the LD bacterium is unexplained but suggests that M. pneumoniae seropositive cases should be evaluated for the possibility of Legionnaires' disease.