The possibility of liver involvement in Mycoplasma pneumoniae pneumonia is still controversial. This study investigated 33 adult patients with serologically confirmed M. pneumoniae community-acquired pneumonia (CAP) (median age 31 years) and 38 patients with bacteraemic Streptococcus pneumoniae CAP (median age 54 years), all without pre-existing liver disease. Serum alanine aminotransferase (ALT) levels were elevated in 12 (36.4%) patients with M. pneumoniae CAP (median 53.5 U/L), and in four (10.5%) patients with S. pneumoniae CAP (median 61 U/L) (p 0.025). In most patients with M. pneumoniae CAP, the elevated ALT levels decreased during macrolide therapy, although this decrease was not significant.