All 18 travelling in the Volkswagen Transporter died, according to police, 16 at the scene of the crash. The lorry driver sustained light injuries but is not thought to be critical.
Police were investigating the cause of the crash which occurred close to Nowe Miasto nad Pilica, some 50 miles south of the Polish capital.
"The utility vehicle was in the left-hand lane and it had a frontal collision with a lorry in thick fog that caused limited visibility. The temperature was slightly below zero," police spokesman Magdalena Siczek-Zalewska said.
"The utility vehicle was not adapted for passengers and had only six seats. People were sitting on sacks and crates. It couldn't have ended otherwise," she added.
Three days of mourning were called in two affected central Polish regions.
The crash is one of the worst in Poland, a country of 38 million, where a poorly maintained road network claims a high number of lives. Last year, 4,572 people perished on Polish roads, according to police statistics.
This September, 415 people died and more than 4,400 were injured in 3,565 road accidents nationwide, statistics said.
Police also detained nearly 13,000 drivers for driving under the influence of alcohol in September.
