Undifferentiated connective tissue disease (UCTD) is a group of systemic autoimmune conditions not fulfilling the classification criteria for a definite connective tissue disease (CTD). While an average of 20% of UCTD patients develop a defined CTD during follow-up, the remaining patients maintain an undefined disease. Since pregnancy is considered to be an important factor that may alter the course of autoimmune diseases, we examined 25 pregnancies in 20 UCTD patients being followed at our unit in order to evaluate: (i) the pregnancy outcome; (ii) whether pregnancy is associated with flares of disease activity; and (iii) whether pregnancy may be a trigger for the development of a defined CTD. Twenty-two pregnancies (88%) were successfully brought to term, while the remaining three (12%) ended in an abortion in the first trimester. Obstetric complications were observed in six out of the 22 successful pregnancies (27%). Six patients (24%) experienced a disease flare during pregnancy or puerperium, one of whom presented a major flare and developed systemic lupus erythematosus. In the other five patients the manifestations at flare were mild and included arthritis, fever and skin rash. The incidence of flares in a control population of non-pregnant UCTD patients over a period of 1 year was 7%. Although UCTD is a mild condition, the risk of flares during pregnancy appears increased and therefore careful monitoring is as necessary as in other CTD patients. Further prospective studies will be necessary to confirm these preliminary observations.