"They were outside the building and waited for the girls to go out for a walk," the director of the public institution, Anna-Maria af Sandeberg, told Swedish news agency TT, without naming the agencies.
The unscrupulous head hunters were familiar with the patients' every day routine and would stop at nothing, she added.
"One of those contacted was in a wheelchair because she was so skinny," af Sandeberg said.
The incident took place several months ago, and the girls' schedules have since been changed.
The director of the Elite Model Management agency in Stockholm, Fredo Kazemi, denounced the methods as "disgusting and unethical."
"I do not think that any large, serious agencies work this way," he told TT.
The Stockholm centre for eating disorders is the largest facility of its kind in Sweden, currently treating some 1,200 in-patients and day patients, most of whom are girls but also boys and adults.
The fashion industry is regularly accused of promoting extreme thinness and working with models that suffer from eating disorders.
Earlier this year, Israel banned models with a BMI (body mass index) level below 18.5 from photo shoots and advertising campaigns.
Other countries have so far only introduced non-binding professional codes of conduct.