The attendance (actual bodies on-site, as opposed to supporting members) at the World Science Fiction Convention over time. The first was held in New York City in 1939. None were held 1942-45 (during World War II).
The first to top 1,000 was 1967 in New York City, with attendance of about 1,500.
The first to top 5,000 was 1980 in Boston, Massachusetts, with attendance of about 5,800.
Attendance peaked in 1984, in Anaheim, California (8,365).
WorldCons held outside North America nearly all show attendance drops. These include:
1957 in London, England (attendance was 268, from the previous year’s 850)
1965 in London (350 from 1964’s 523)
1970 in Heidelberg, Germany (620, previously 1,534)
1975 in Melbourne, Australia (606, previously 3,587)
1979 in Brighton, England (3,114, previously 4,700)
1985 in Melbourne, Australia (1,599, previously 8,365)
1987 in Brighton, England (4,009, previously 5,811)
1990 in The Hague, The Netherlands (3,580, previously 6,837)
1995 in Glasgow, Scotland (4,173, previously 3,570)
1999 in Melbourne, Australia (1,626, previously 6,572)
2005 in Glasgow, Scotland (4,115, previously 5,651)
2007 in Yokohama, Japan (2,788, previously 5,738)
The 1994 WorldCon, in Winnipeg, Manitoba, Canada, was the only North American WorldCon to be followed by one outside North America at which attendance was greater. The next non-North American WorldCon will be next year’s, again in Melbourne, Australia.