Three inexperienced operators (2 midwives and one obstetric registrar) were given ten sessions of training in ovarian follicular scanning. During each session, five patients were examined jointly. Once the inexperienced observers had received sufficient training the prospective study was started. Twenty measurements by each inexperienced observer were compared to measurements made by an experienced observer. In 17% of measurements the error was greater than 3 mm, the errors being evenly distributed among the three observers. After a further five sessions of instructions, another set of sixty measurements was generated: 93% of these measurements correlated exactly among the inexperienced operators and between the inexperienced and the experienced operator. This study suggests that after a supervised training of 15 sessions, obstetricians and midwives with prior obstetric scanning experience can produce reliable ovarian follicular measurements.