When 3ts, a managing agent of the portfolio of a private equity giant 3i in the Central Eastern Europe, was considering an investment into a Bucharest-based health care company Unirea Medical Centre (later Regina Maria), they invited Dr. Rantanen to perform a medical due diligence analysis of the potential target. After the purchase, the company invited him to stay with the company as a member of its board of directors. During his three-year term as a board member, Dr. Rantanen helped the company to grow into one of the largest private health care providers in Romania.