Abstract: The aim is to determine the factors contributing to outpatient follow up default rate (attrition) in a prospective study on foetal malnutrition in Nigeria and proffer suggestions on how to reduce it. This is a study of outpatient follow up attendance attrition (attendance default rate) in a study of Foetal Malnutrition in consecutive mothers who delivered singleton, term, live babies between January and August 2002 at the Wesley Guild Hospital, Ilesa, Nigeria. Foetal malnutrition was diagnosed using Clinical Assessment of Fetal Nutritional Status (CANS) and the score (CANSCORE) as adapted by Metcoff (1994). The mothers of the babies were informed that they would be required to attend the follow up clinic after discharge. The present research is the report on the rates of default from clinic attendance after discharge presented in relation to certain maternal and baby factors which had been elicited in the research proforma prior to the clinical assessment exercise. These factors were the socio-economic class, parity, age, attendance for antenatal care of the mothers and the presence or absence of foetal malnutrition or illnesses compelling hospital admission in the babies. Of the 473 studied and expected to be followed up, 89 [18.8%] had FM. Three hundred and fifteen (66.6%) and 249 (52.6%) were the attendance in the follow up clinic at the first and fourth weeks respectively. The 224 (47.4%) lost to follow up at the 4th week were examined in relation to the factors listed under methodology. Attrition rate at the 4 weeks follow up was higher among the mothers who had no antenatal care compared with those who did ( ² = 11.3, p = 0.001), among teenage than older mothers ( ² = 3.1, p = 0.08), among primiparous than multiparous mothers ( ² = 2.5, p = 0.1) and among lower than higher social class mothers ( ² = 8.0, p = 0.005). Attrition was also higher among mothers of female than male sex babies ( ² = 4.5, p = 0.03), mothers of babies without foetal malnutrition than those with FM (p = 0.5), among clinically well than ill babies requiring admissions ( ² = 39.7, p = 0.000). To reduce the frequency of attrition in a research study, better education of subjects and enlistment of the services of social worker/health visitors should be undertaken. Researchers in the developing nations should make generous allowance for attrition in calculating the number of the subjects to be recruited. This may be higher than what obtains in the developed nations. Research work should be made relevant to subjects need and planned in a way that subject participation would not overstretch their resources. Improvement in infrastructure including transportation and road network would also assist research activities.