We investigated 627 patients who within a period of 2 1/2 years had had a rubelliform rash and/or symptoms of arthritis and arthralgia. Sera from these patients were investigated for evidence of rubella, human parvovirus B19 (HPV), and measles infection with methods to detect specific IgM and IgG antibodies. Complement fixation tests were used to screen for a wide range of other infectious agents. We detected 229 cases of rubella, 43 cases of HPV infection, 7 cases of measles, and 9 cases of infection by various other aetiological agents. This left a large proportion of rubelliform rashes, 54% (339 cases), whose aetiology was unknown. This study confirmed that the diagnosis of rubella on clinical grounds alone is unreliable. Many (6.8%) of the rashes in the study were due to HPV infection, and the seasonal incidence was the same as for rubella. There was considerable overlap between the features of rubella and HPV infections, although in adults arthralgia occurred more frequently in HPV infections than in rubella. In all cases HPV infection was self-limiting, although, as in rubella, symptoms can be prolonged and one adult's disease lasted almost 9 months. Purpura was noted in only one patient with HPV infection. In this study three patients had HPV infection during pregnancy. Two patients spontaneously aborted one month later. The third patient progressed to full term and delivered a healthy baby.