As someone who has followed the District's foster care problem closely for some time I must write concerning what struck me as a rather material omission in both Laura Sessions Stepp's article {"A Year of Frustration for D.C. Foster Care," Metro, Dec. 31} and a Jan. 2 editorial {"Saving Children"}.

Both pieces left the strong impression that even in the wake of The Post's series on the boarder babies {front page, Sept. 10-12, 1989} only a handful of D.C. residents were willing to sacrifice the time to complete the necessary training and become foster parents. Laura Stepp's article stated: ''Of the 400 people who telephoned the city wanting to become foster parents after The Post series, 237 enrolled in classes, and 42 completed the training.'' The editorial lamented the fact that ''only 290 Washington families -- only 290 -- are willing to provide foster care.''

What the article and editorial fail to point out is that the low number of people who competed training was driven not by lack of a compassionate response on the part of the District's citizens, but by a failure on the part of the Adoption and Placement Resources Branch of the Department of Human Services to take wise advantage of the media attention focused on the problem and expand their classes to accommodate those people who did respond.

I was one of many who attended the Sept. 12 orientation to learn more about becoming a foster parent. What I learned was that the branch holds those courses only four times a year and by design trains only a total of 200 foster families in the four sessions. If I did not get in on the eight-week series beginning in September, I would have to wait until the next one began in January. There were easily twice and perhaps three times the number of people attending the introductory meeting as would fill the open slots in the class.

After a two-hour presentation of what would be covered in the next eight sessions, we were all given a number to call the next morning in order to register on a first-come, first-served basis. I called at 9:30 a.m., and the class was full. I was put on a waiting list and told that I would be contacted before the January classes began. I received that call last month. Because I am still living in a one-bedroom apartment, and the placement branch requires a separate bedroom for the child (a sleeper sofa in the living room does not meet the standard), I surrendered the slot in the class until my living situation changes.

I am not suggesting that it would have been an easy task to absorb more trainees in the September session. The women teaching the course -- quite competently, I might add -- were already working two evenings a week to train the number of prospective foster parents that they felt they could get to know sufficiently to entrust children to when the course ended. Certainly no one can fault that reasoning. But in light of the overwhelming need and the number of people who have responded, surely something could have been done to bring the two together -- perhaps a daytime course or even a weekend seminar.

It would not have been easy, but it should have been done.

CATHRYN M. DORSEY Washington